Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections.

1.1 Overview

Briefly, a boot loader is the first software program that runs when
a computer starts. It is responsible for loading and transferring
control to an operating system kernel software (such as Linux or
GNU Mach). The kernel, in turn, initializes the rest of the operating
system (e.g. a GNU system).

GNU GRUB is a very powerful boot loader, which can load a wide variety
of free operating systems, as well as proprietary operating systems with
chain-loading1. GRUB is designed to
address the complexity of booting a personal computer; both the
program and this manual are tightly bound to that computer platform,
although porting to other platforms may be addressed in the future.

One of the important features in GRUB is flexibility; GRUB understands
filesystems and kernel executable formats, so you can load an arbitrary
operating system the way you like, without recording the physical
position of your kernel on the disk. Thus you can load the kernel
just by specifying its file name and the drive and partition where the
kernel resides.

When booting with GRUB, you can use either a command-line interface
(see Command-line interface), or a menu interface (see Menu interface). Using the command-line interface, you type the drive
specification and file name of the kernel manually. In the menu
interface, you just select an OS using the arrow keys. The menu is
based on a configuration file which you prepare beforehand
(see Configuration). While in the menu, you can switch to the
command-line mode, and vice-versa. You can even edit menu entries
before using them.

In the following chapters, you will learn how to specify a drive, a
partition, and a file name (see Naming convention) to GRUB, how to
install GRUB on your drive (see Installation), and how to boot your
OSes (see Booting), step by step.

1.2 History of GRUB

GRUB originated in 1995 when Erich Boleyn was trying to boot the GNU
Hurd with the University of Utah’s Mach 4 microkernel (now known as GNU
Mach). Erich and Brian Ford designed the Multiboot Specification
(see Motivation in The Multiboot
Specification), because they were determined not to add to the large
number of mutually-incompatible PC boot methods.

Erich then began modifying the FreeBSD boot loader so that it would
understand Multiboot. He soon realized that it would be a lot easier
to write his own boot loader from scratch than to keep working on the
FreeBSD boot loader, and so GRUB was born.

Erich added many features to GRUB, but other priorities prevented him
from keeping up with the demands of its quickly-expanding user base. In
1999, Gordon Matzigkeit and Yoshinori K. Okuji adopted GRUB as an
official GNU package, and opened its development by making the latest
sources available via anonymous CVS. See Obtaining and Building GRUB, for more information.

Over the next few years, GRUB was extended to meet many needs, but it
quickly became clear that its design was not keeping up with the extensions
being made to it, and we reached the point where it was very difficult to
make any further changes without breaking existing features. Around 2002,
Yoshinori K. Okuji started work on PUPA (Preliminary Universal Programming
Architecture for GNU GRUB), aiming to rewrite the core of GRUB to make it
cleaner, safer, more robust, and more powerful. PUPA was eventually renamed
to GRUB 2, and the original version of GRUB was renamed to GRUB Legacy.
Small amounts of maintenance continued to be done on GRUB Legacy, but the
last release (0.97) was made in 2005 and at the time of writing it seems
unlikely that there will be another.

By around 2007, GNU/Linux distributions started to use GRUB 2 to limited
extents, and by the end of 2009 multiple major distributions were installing
it by default.

1.3 Differences from previous versions

GRUB 2 is a rewrite of GRUB (see History), although it shares many
characteristics with the previous version, now known as GRUB Legacy. Users
of GRUB Legacy may need some guidance to find their way around this new
version.

The configuration file has a new name (grub.cfg rather than
menu.lst or grub.conf), new syntax (see Configuration) and
many new commands (see Commands). Configuration cannot be copied over
directly, although most GRUB Legacy users should not find the syntax too
surprising.

The configuration file is now written in something closer to a full
scripting language: variables, conditionals, and loops are available.

A small amount of persistent storage is available across reboots, using the
save_env and load_env commands in GRUB and the
grub-editenv utility. This is not available in all configurations
(see Environment block).

GRUB 2 has more reliable ways to find its own files and those of target
kernels on multiple-disk systems, and has commands (see search) to find
devices using file system labels or Universally Unique Identifiers (UUIDs).

GRUB 2 is available for several other types of system in addition to the PC
BIOS systems supported by GRUB Legacy: PC EFI, PC coreboot, PowerPC, SPARC,
and MIPS Lemote Yeeloong are all supported.

Many more file systems are supported, including but not limited to ext4,
HFS+, and NTFS.

GRUB 2 can read files directly from LVM and RAID devices.

A graphical terminal and a graphical menu system are available.

GRUB 2’s interface can be translated, including menu entry names.

The image files (see Images) that make up GRUB have been reorganised;
Stage 1, Stage 1.5, and Stage 2 are no more.

GRUB 2 puts many facilities in dynamically loaded modules, allowing the core
image to be smaller, and allowing the core image to be built in more
flexible ways.

Except for specific compatibility modes (chain-loading and the Linux
piggyback format), all kernels will be started in much the same
state as in the Multiboot Specification. Only kernels loaded at 1 megabyte
or above are presently supported. Any attempt to load below that
boundary will simply result in immediate failure and an error message
reporting the problem.

In addition to the requirements above, GRUB has the following features
(note that the Multiboot Specification doesn’t require all the features
that GRUB supports):

Recognize multiple executable formats

Support many of the a.out variants plus ELF. Symbol
tables are also loaded.

Support non-Multiboot kernels

Support many of the various free 32-bit kernels that lack Multiboot
compliance (primarily FreeBSD, NetBSD2,
OpenBSD, and Linux). Chain-loading of other boot loaders is also
supported.

Load multiples modules

Fully support the Multiboot feature of loading multiple modules.

Load a configuration file

Support a human-readable text configuration file with preset boot
commands. You can also load another configuration file dynamically and
embed a preset configuration file in a GRUB image file. The list of
commands (see Commands) are a superset of those supported on the
command-line. An example configuration file is provided in
Configuration.

Provide a menu interface

A menu interface listing preset boot commands, with a programmable
timeout, is available. There is no fixed limit on the number of boot
entries, and the current implementation has space for several hundred.

Have a flexible command-line interface

A fairly flexible command-line interface, accessible from the menu,
is available to edit any preset commands, or write a new boot command
set from scratch. If no configuration file is present, GRUB drops to
the command-line.

The list of commands (see Commands) are a subset of those supported
for configuration files. Editing commands closely resembles the Bash
command-line (see Command Line Editing in Bash Features), with TAB-completion of commands,
devices, partitions, and files in a directory depending on context.

Can decompress files which were compressed by gzip or
xz3. This function is both automatic and transparent to the user
(i.e. all functions operate upon the uncompressed contents of the specified
files). This greatly reduces a file size and loading time, a
particularly great benefit for floppies.4

It is conceivable that some kernel modules should be loaded in a
compressed state, so a different module-loading command can be specified
to avoid uncompressing the modules.

Access data on any installed device

Support reading data from any or all floppies or hard disk(s) recognized
by the BIOS, independent of the setting of the root device.

Be independent of drive geometry translations

Unlike many other boot loaders, GRUB makes the particular drive
translation irrelevant. A drive installed and running with one
translation may be converted to another translation without any adverse
effects or changes in GRUB’s configuration.

Detect all installed RAM

GRUB can generally find all the installed RAM on a PC-compatible
machine. It uses an advanced BIOS query technique for finding all
memory regions. As described on the Multiboot Specification (see Motivation in The Multiboot
Specification), not all kernels make use of this information, but GRUB
provides it for those who do.

Support Logical Block Address mode

In traditional disk calls (called CHS mode), there is a geometry
translation problem, that is, the BIOS cannot access over 1024
cylinders, so the accessible space is limited to at least 508 MB and to
at most 8GB. GRUB can’t universally solve this problem, as there is no
standard interface used in all machines. However, several newer machines
have the new interface, Logical Block Address (LBA) mode. GRUB
automatically detects if LBA mode is available and uses it if
available. In LBA mode, GRUB can access the entire disk.

Support network booting

GRUB is basically a disk-based boot loader but also has network
support. You can load OS images from a network by using the TFTP
protocol.

Support remote terminals

To support computers with no console, GRUB provides remote terminal
support, so that you can control GRUB from a remote host. Only serial
terminal support is implemented at the moment.

1.5 The role of a boot loader

The following is a quotation from Gordon Matzigkeit, a GRUB fanatic:

Some people like to acknowledge both the operating system and kernel when
they talk about their computers, so they might say they use
“GNU/Linux” or “GNU/Hurd”. Other people seem to think that the
kernel is the most important part of the system, so they like to call
their GNU operating systems “Linux systems.”

I, personally, believe that this is a grave injustice, because the
boot loader is the most important software of all. I used to
refer to the above systems as either “LILO”5 or “GRUB”
systems.

Unfortunately, nobody ever understood what I was talking about; now I
just use the word “GNU” as a pseudonym for GRUB.

So, if you ever hear people talking about their alleged “GNU” systems,
remember that they are actually paying homage to the best boot loader
around… GRUB!

We, the GRUB maintainers, do not (usually) encourage Gordon’s level of
fanaticism, but it helps to remember that boot loaders deserve
recognition. We hope that you enjoy using GNU GRUB as much as we did
writing it.

2 Naming convention

The device syntax used in GRUB is a wee bit different from what you may
have seen before in your operating system(s), and you need to know it so
that you can specify a drive/partition.

Look at the following examples and explanations:

(fd0)

First of all, GRUB requires that the device name be enclosed with
‘(’ and ‘)’. The ‘fd’ part means that it is a floppy
disk. The number ‘0’ is the drive number, which is counted from
zero. This expression means that GRUB will use the whole floppy
disk.

(hd0,msdos2)

Here, ‘hd’ means it is a hard disk drive. The first integer
‘0’ indicates the drive number, that is, the first hard disk,
the string ‘msdos’ indicates the partition scheme, while
the second integer, ‘2’, indicates the partition number (or the
PC slice number in the BSD terminology). The partition numbers are
counted from one, not from zero (as was the case in previous
versions of GRUB). This expression means the second partition of the
first hard disk drive. In this case, GRUB uses one partition of the
disk, instead of the whole disk.

(hd0,msdos5)

This specifies the first extended partition of the first hard disk
drive. Note that the partition numbers for extended partitions are
counted from ‘5’, regardless of the actual number of primary
partitions on your hard disk.

(hd1,msdos1,bsd1)

This means the BSD ‘a’ partition on first PC slice number
of the second hard disk.

Of course, to actually access the disks or partitions with GRUB, you
need to use the device specification in a command, like ‘set
root=(fd0)’ or ‘parttool (hd0,msdos3) hidden-’. To help you find out
which number specifies a partition you want, the GRUB command-line
(see Command-line interface) options have argument
completion. This means that, for example, you only need to type

set root=(

followed by a TAB, and GRUB will display the list of drives,
partitions, or file names. So it should be quite easy to determine the
name of your target partition, even with minimal knowledge of the
syntax.

Note that GRUB does not distinguish IDE from SCSI - it simply
counts the drive numbers from zero, regardless of their type. Normally,
any IDE drive number is less than any SCSI drive number, although that
is not true if you change the boot sequence by swapping IDE and SCSI
drives in your BIOS.

Now the question is, how to specify a file? Again, consider an
example:

(hd0,msdos1)/vmlinuz

This specifies the file named ‘vmlinuz’, found on the first
partition of the first hard disk drive. Note that the argument
completion works with file names, too.

That was easy, admit it. Now read the next chapter, to find out how to
actually install GRUB on your drive.

4 Installation

In order to install GRUB as your boot loader, you need to first
install the GRUB system and utilities under your UNIX-like operating
system (see Obtaining and Building GRUB). You can do this either
from the source tarball, or as a package for your OS.

After you have done that, you need to install the boot loader on a
drive (floppy or hard disk) by using the utility
grub-install (see Invoking grub-install) on a UNIX-like OS.

GRUB comes with boot images, which are normally put in the directory
/usr/lib/grub/<cpu>-<platform> (for BIOS-based machines
/usr/lib/grub/i386-pc). Hereafter, the directory where GRUB images are
initially placed (normally /usr/lib/grub/<cpu>-<platform>) will be
called the image directory, and the directory where the boot
loader needs to find them (usually /boot) will be called
the boot directory.

4.1 Installing GRUB using grub-install

For information on where GRUB should be installed on PC BIOS platforms,
see BIOS installation.

In order to install GRUB under a UNIX-like OS (such
as GNU), invoke the program grub-install (see Invoking grub-install) as the superuser (root).

The usage is basically very simple. You only need to specify one
argument to the program, namely, where to install the boot loader. The
argument has to be either a device file (like ‘/dev/hda’).
For example, under Linux the following will install GRUB into the MBR
of the first IDE disk:

# grub-install /dev/sda

Likewise, under GNU/Hurd, this has the same effect:

# grub-install /dev/hd0

But all the above examples assume that GRUB should put images under
the /boot directory. If you want GRUB to put images under a directory
other than /boot, you need to specify the option
--boot-directory. The typical usage is that you create a GRUB
boot floppy with a filesystem. Here is an example:

Some BIOSes have a bug of exposing the first partition of a USB drive as a
floppy instead of exposing the USB drive as a hard disk (they call it
“USB-FDD” boot). In such cases, you need to install like this:

This install doesn’t conflict with standard install as long as they are in
separate directories.

Note that grub-install is actually just a shell script and the
real task is done by other tools such as grub-mkimage. Therefore,
you may run those commands directly to install GRUB, without using
grub-install. Don’t do that, however, unless you are very familiar
with the internals of GRUB. Installing a boot loader on a running OS may be
extremely dangerous.

On EFI systems for fixed disk install you have to mount EFI System Partition.
If you mount it at /boot/efi then you don’t need any special arguments:

# grub-install

Otherwise you need to specify where your EFI System partition is mounted:

# grub-install --efi-directory=/mnt/efi

For removable installs you have to use --removable and specify both
--boot-directory and --efi-directory:

4.2 Making a GRUB bootable CD-ROM

GRUB supports the no emulation mode in the El Torito
specification6. This means that you can use the whole CD-ROM
from GRUB and you don’t have to make a floppy or hard disk image file,
which can cause compatibility problems.

For booting from a CD-ROM, GRUB uses a special image called
cdboot.img, which is concatenated with core.img. The
core.img used for this should be built with at least the
‘iso9660’ and ‘biosdisk’ modules. Your bootable CD-ROM will
usually also need to include a configuration file grub.cfg and some
other GRUB modules.

You will often need to include other files in your image. To do this, first
make a top directory for the bootable image, say, ‘iso’:

$ mkdir iso

Make a directory for GRUB:

$ mkdir -p iso/boot/grub

If desired, make the config file grub.cfg under iso/boot/grub
(see Configuration), and copy any files and directories for the disc to the
directory iso/.

Finally, make the image:

$ grub-mkrescue -o grub.iso iso

This produces a file named grub.iso, which then can be burned
into a CD (or a DVD), or written to a USB mass storage device.

The root device will be set up appropriately on entering your
grub.cfg configuration file, so you can refer to file names on the CD
without needing to use an explicit device name. This makes it easier to
produce rescue images that will work on both optical drives and USB mass
storage devices.

4.3 The map between BIOS drives and OS devices

If the device map file exists, the GRUB utilities (grub-probe,
etc.) read it to map BIOS drives to OS devices. This file consists of lines
like this:

(device) file

device is a drive specified in the GRUB syntax (see Device syntax), and file is an OS file, which is normally a device file.

Historically, the device map file was used because GRUB device names had to
be used in the configuration file, and they were derived from BIOS drive
numbers. The map between BIOS drives and OS devices cannot always be
guessed correctly: for example, GRUB will get the order wrong if you
exchange the boot sequence between IDE and SCSI in your BIOS.

Unfortunately, even OS device names are not always stable. Modern versions
of the Linux kernel may probe drives in a different order from boot to boot,
and the prefix (/dev/hd* versus /dev/sd*) may change depending
on the driver subsystem in use. As a result, the device map file required
frequent editing on some systems.

GRUB avoids this problem nowadays by using UUIDs or file system labels when
generating grub.cfg, and we advise that you do the same for any
custom menu entries you write. If the device map file does not exist, then
the GRUB utilities will assume a temporary device map on the fly. This is
often good enough, particularly in the common case of single-disk systems.

However, the device map file is not entirely obsolete yet, and it is
used for overriding when current environment is different from the one on boot.
Most common case is if you use a partition or logical volume as a disk for
virtual machine. You can put any comments in the file if needed,
as the GRUB utilities assume that a line is just a comment if
the first character is ‘#’.

4.4 BIOS installation

MBR

The partition table format traditionally used on PC BIOS platforms is called
the Master Boot Record (MBR) format; this is the format that allows up to
four primary partitions and additional logical partitions. With this
partition table format, there are two ways to install GRUB: it can be
embedded in the area between the MBR and the first partition (called by
various names, such as the "boot track", "MBR gap", or "embedding area", and
which is usually at least 31 KiB), or the core image can be installed in a
file system and a list of the blocks that make it up can be stored in the
first sector of that partition.

Each of these has different problems. There is no way to reserve space in
the embedding area with complete safety, and some proprietary software is
known to use it to make it difficult for users to work around licensing
restrictions; and systems are sometimes partitioned without leaving enough
space before the first partition. On the other hand, installing to a
filesystem means that GRUB is vulnerable to its blocks being moved around by
filesystem features such as tail packing, or even by aggressive fsck
implementations, so this approach is quite fragile; and this approach can
only be used if the /boot filesystem is on the same disk that the
BIOS boots from, so that GRUB does not have to rely on guessing BIOS drive
numbers.

The GRUB development team generally recommends embedding GRUB before the
first partition, unless you have special requirements. You must ensure that
the first partition starts at least 31 KiB (63 sectors) from the start of
the disk; on modern disks, it is often a performance advantage to align
partitions on larger boundaries anyway, so the first partition might start 1
MiB from the start of the disk.

GPT

Some newer systems use the GUID Partition Table (GPT) format. This was
specified as part of the Extensible Firmware Interface (EFI), but it can
also be used on BIOS platforms if system software supports it; for example,
GRUB and GNU/Linux can be used in this configuration. With this format, it
is possible to reserve a whole partition for GRUB, called the BIOS Boot
Partition. GRUB can then be embedded into that partition without the risk
of being overwritten by other software and without being contained in a
filesystem which might move its blocks around.

When creating a BIOS Boot Partition on a GPT system, you should make sure
that it is at least 31 KiB in size. (GPT-formatted disks are not usually
particularly small, so we recommend that you make it larger than the bare
minimum, such as 1 MiB, to allow plenty of room for growth.) You must also
make sure that it has the proper partition type. Using GNU Parted, you can
set this using a command such as the following:

# parted /dev/disk set partition-number bios_grub on

If you are using gdisk, set the partition type to ‘0xEF02’. With
partitioning programs that require setting the GUID directly, it should be
‘21686148-6449-6e6f-744e656564454649’.

Caution: Be very careful which partition you select! When GRUB
finds a BIOS Boot Partition during installation, it will automatically
overwrite part of it. Make sure that the partition does not contain any
other data.

5.1 How to boot operating systems

GRUB has two distinct boot methods. One of the two is to load an
operating system directly, and the other is to chain-load another boot
loader which then will load an operating system actually. Generally
speaking, the former is more desirable, because you don’t need to
install or maintain other boot loaders and GRUB is flexible enough to
load an operating system from an arbitrary disk/partition. However,
the latter is sometimes required, since GRUB doesn’t support all the
existing operating systems natively.

5.1.1 How to boot an OS directly with GRUB

Multiboot (see Motivation in The Multiboot Specification) is the native format supported by GRUB.
For the sake of convenience, there is also support for Linux, FreeBSD,
NetBSD and OpenBSD. If you want to boot other operating systems, you
will have to chain-load them (see Chain-loading).

5.1.2 Chain-loading an OS

Operating systems that do not support Multiboot and do not have specific
support in GRUB (specific support is available for Linux, FreeBSD, NetBSD
and OpenBSD) must be chain-loaded, which involves loading another boot
loader and jumping to it in real mode.

The chainloader command (see chainloader) is used to set this
up. It is normally also necessary to load some GRUB modules and set the
appropriate root device. Putting this together, we get something like this,
for a Windows system on the first partition of the first hard disk:

5.2 Loopback booting

GRUB is able to read from an image (be it one of CD or HDD) stored on
any of its accessible storages (refer to see loopback command).
However the OS itself should be able to find its root. This usually
involves running a userspace program running before the real root
is discovered. This is achieved by GRUB loading a specially made
small image and passing it as ramdisk to the kernel. This is achieved
by commands kfreebsd_module, knetbsd_module_elf,
kopenbsd_ramdisk, initrd (see initrd),
initrd16 (see initrd), multiboot_module,
multiboot2_module or xnu_ramdisk
depending on the loader. Note that for knetbsd the image must be put
inside miniroot.kmod and the whole miniroot.kmod has to be loaded. In
kopenbsd payload this is disabled by default. Aditionally behaviour of
initial ramdisk depends on command line options. Several distributors provide
the image for this purpose or it’s integrated in their standard ramdisk and
activated by special option. Consult your kernel and distribution manual for
more details. Other loaders like appleloader, chainloader (BIOS, EFI, coreboot),
freedos, ntldr and plan9 provide no possibility of loading initial ramdisk and
as far as author is aware the payloads in question don’t support either initial
ramdisk or discovering loopback boot in other way and as such not bootable this
way. Please consider alternative boot methods like copying all files
from the image to actual partition. Consult your OS documentation for
more details

Caution: If you use an initrd and specify the ‘mem=’
option to the kernel to let it use less than actual memory size, you
will also have to specify the same memory size to GRUB. To let GRUB know
the size, run the command uppermembefore loading the
kernel. See uppermem, for more information.

5.3.3 NetBSD

Booting a NetBSD kernel from GRUB is also relatively easy: first set
GRUB’s root device, then load the kernel and the modules, and finally
run boot.

Set GRUB’s root device to the partition holding the NetBSD root file
system. For a disk with a NetBSD disk label, this is usually the first
partition (a:). In that case, and assuming that the partition is on the
first hard disk, set GRUB’s root device as follows:

grub> insmod part_bsd
grub> set root=(hd0,netbsd1)

For a disk with a GUID Partition Table (GPT), and assuming that the
NetBSD root partition is the third GPT partition, do this:

grub> insmod part_gpt
grub> set root=(hd0,gpt3)

Load the kernel using the command knetbsd:

grub> knetbsd /netbsd

Various options may be given to knetbsd. These options are,
for the most part, the same as in the NetBSD boot loader. For instance,
to boot the system in single-user mode and with verbose messages, do
this:

grub> knetbsd /netbsd -s -v

If needed, load kernel modules with the command
knetbsd_module_elf. A typical example is the module for the
root file system:

5.3.4 DOS/Windows

GRUB cannot boot DOS or Windows directly, so you must chain-load them
(see Chain-loading). However, their boot loaders have some critical
deficiencies, so it may not work to just chain-load them. To overcome
the problems, GRUB provides you with two helper functions.

If you have installed DOS (or Windows) on a non-first hard disk, you
have to use the disk swapping technique, because that OS cannot boot
from any disks but the first one. The workaround used in GRUB is the
command drivemap (see drivemap), like this:

drivemap -s (hd0) (hd1)

This performs a virtual swap between your first and second hard
drive.

Caution: This is effective only if DOS (or Windows) uses BIOS
to access the swapped disks. If that OS uses a special driver for the
disks, this probably won’t work.

Another problem arises if you installed more than one set of DOS/Windows
onto one disk, because they could be confused if there are more than one
primary partitions for DOS/Windows. Certainly you should avoid doing
this, but there is a solution if you do want to do so. Use the partition
hiding/unhiding technique.

If GRUB hides a DOS (or Windows) partition (see parttool), DOS (or
Windows) will ignore the partition. If GRUB unhides a DOS (or Windows)
partition, DOS (or Windows) will detect the partition. Thus, if you have
installed DOS (or Windows) on the first and the second partition of the
first hard disk, and you want to boot the copy on the first partition, do
the following:

6.1 Simple configuration handling

The program grub-mkconfig (see Invoking grub-mkconfig)
generates grub.cfg files suitable for most cases. It is suitable for
use when upgrading a distribution, and will discover available kernels and
attempt to generate menu entries for them.

grub-mkconfig does have some limitations. While adding extra
custom menu entries to the end of the list can be done by editing
/etc/grub.d/40_custom or creating /boot/grub/custom.cfg,
changing the order of menu entries or changing their titles may require
making complex changes to shell scripts stored in /etc/grub.d/. This
may be improved in the future. In the meantime, those who feel that it
would be easier to write grub.cfg directly are encouraged to do so
(see Booting, and Shell-like scripting), and to disable any system
provided by their distribution to automatically run grub-mkconfig.

The file /etc/default/grub controls the operation of
grub-mkconfig. It is sourced by a shell script, and so must be
valid POSIX shell input; normally, it will just be a sequence of
‘KEY=value’ lines, but if the value contains spaces or other special
characters then it must be quoted. For example:

GRUB_TERMINAL_INPUT="console serial"

Valid keys in /etc/default/grub are as follows:

‘GRUB_DEFAULT’

The default menu entry. This may be a number, in which case it identifies
the Nth entry in the generated menu counted from zero, or the title of a
menu entry, or the special string ‘saved’. Using the id may be
useful if you want to set a menu entry as the default even though there may
be a variable number of entries before it.

Previously it was documented the way to use entry title. While this still
works it’s not recommended since titles often contain unstable device names
and may be translated

If you set this to ‘saved’, then the default menu entry will be that
saved by ‘GRUB_SAVEDEFAULT’ or grub-set-default. This relies on
the environment block, which may not be available in all situations
(see Environment block).

The default is ‘0’.

‘GRUB_SAVEDEFAULT’

If this option is set to ‘true’, then, when an entry is selected, save
it as a new default entry for use by future runs of GRUB. This is only
useful if ‘GRUB_DEFAULT=saved’; it is a separate option because
‘GRUB_DEFAULT=saved’ is useful without this option, in conjunction with
grub-set-default. Unset by default.
This option relies on the environment block, which may not be available in
all situations (see Environment block).

‘GRUB_TIMEOUT’

Boot the default entry this many seconds after the menu is displayed, unless
a key is pressed. The default is ‘5’. Set to ‘0’ to boot
immediately without displaying the menu, or to ‘-1’ to wait
indefinitely.

If ‘GRUB_TIMEOUT_STYLE’ is set to ‘countdown’ or ‘hidden’,
the timeout is instead counted before the menu is displayed.

‘GRUB_TIMEOUT_STYLE’

If this option is unset or set to ‘menu’, then GRUB will display the
menu and then wait for the timeout set by ‘GRUB_TIMEOUT’ to expire
before booting the default entry. Pressing a key interrupts the timeout.

If this option is set to ‘countdown’ or ‘hidden’, then, before
displaying the menu, GRUB will wait for the timeout set by
‘GRUB_TIMEOUT’ to expire. If ESC is pressed during that time, it
will display the menu and wait for input. If a hotkey associated with a
menu entry is pressed, it will boot the associated menu entry immediately.
If the timeout expires before either of these happens, it will boot the
default entry. In the ‘countdown’ case, it will show a one-line
indication of the remaining time.

‘GRUB_DEFAULT_BUTTON’

‘GRUB_TIMEOUT_BUTTON’

‘GRUB_TIMEOUT_STYLE_BUTTON’

‘GRUB_BUTTON_CMOS_ADDRESS’

Variants of the corresponding variables without the ‘_BUTTON’ suffix,
used to support vendor-specific power buttons. See Vendor power-on keys.

‘GRUB_DISTRIBUTOR’

Set by distributors of GRUB to their identifying name. This is used to
generate more informative menu entry titles.

‘spkmodem’ is useful when no serial port is available. Connect the output
of sending system (where GRUB is running) to line-in of receiving system
(usually developer machine).
On receiving system compile ‘spkmodem-recv’ from
‘util/spkmodem-recv.c’ and run:

parecord --channels=1 --rate=48000 --format=s16le | ./spkmodem-recv

The default is to use the platform’s native terminal output.

‘GRUB_TERMINAL’

If this option is set, it overrides both ‘GRUB_TERMINAL_INPUT’ and
‘GRUB_TERMINAL_OUTPUT’ to the same value.

‘GRUB_SERIAL_COMMAND’

A command to configure the serial port when using the serial console.
See serial. Defaults to ‘serial’.

‘GRUB_CMDLINE_LINUX’

Command-line arguments to add to menu entries for the Linux kernel.

‘GRUB_CMDLINE_LINUX_DEFAULT’

Unless ‘GRUB_DISABLE_RECOVERY’ is set to ‘true’, two menu
entries will be generated for each Linux kernel: one default entry and one
entry for recovery mode. This option lists command-line arguments to add
only to the default menu entry, after those listed in
‘GRUB_CMDLINE_LINUX’.

‘GRUB_CMDLINE_NETBSD’

‘GRUB_CMDLINE_NETBSD_DEFAULT’

As ‘GRUB_CMDLINE_LINUX’ and ‘GRUB_CMDLINE_LINUX_DEFAULT’, but for
NetBSD.

‘GRUB_CMDLINE_GNUMACH’

As ‘GRUB_CMDLINE_LINUX’, but for GNU Mach.

‘GRUB_CMDLINE_XEN’

‘GRUB_CMDLINE_XEN_DEFAULT’

The values of these options are passed to Xen hypervisor Xen menu entries,
for all respectively normal entries.

‘GRUB_CMDLINE_LINUX_XEN_REPLACE’

‘GRUB_CMDLINE_LINUX_XEN_REPLACE_DEFAULT’

The values of these options replace the values of ‘GRUB_CMDLINE_LINUX’
and ‘GRUB_CMDLINE_LINUX_DEFAULT’ for Linux and Xen menu entries.

‘GRUB_DISABLE_LINUX_UUID’

Normally, grub-mkconfig will generate menu entries that use
universally-unique identifiers (UUIDs) to identify the root filesystem to
the Linux kernel, using a ‘root=UUID=...’ kernel parameter. This is
usually more reliable, but in some cases it may not be appropriate. To
disable the use of UUIDs, set this option to ‘true’.

‘GRUB_DISABLE_RECOVERY’

If this option is set to ‘true’, disable the generation of recovery
mode menu entries.

‘GRUB_VIDEO_BACKEND’

If graphical video support is required, either because the ‘gfxterm’
graphical terminal is in use or because ‘GRUB_GFXPAYLOAD_LINUX’ is set,
then grub-mkconfig will normally load all available GRUB video
drivers and use the one most appropriate for your hardware. If you need to
override this for some reason, then you can set this option.

After grub-install has been run, the available video drivers are
listed in /boot/grub/video.lst.

‘GRUB_GFXMODE’

Set the resolution used on the ‘gfxterm’ graphical terminal. Note that
you can only use modes which your graphics card supports via VESA BIOS
Extensions (VBE), so for example native LCD panel resolutions may not be
available. The default is ‘auto’, which tries to select a preferred
resolution. See gfxmode.

‘GRUB_BACKGROUND’

Set a background image for use with the ‘gfxterm’ graphical terminal.
The value of this option must be a file readable by GRUB at boot time, and
it must end with .png, .tga, .jpg, or .jpeg.
The image will be scaled if necessary to fit the screen.

‘GRUB_THEME’

Set a theme for use with the ‘gfxterm’ graphical terminal.

‘GRUB_GFXPAYLOAD_LINUX’

Set to ‘text’ to force the Linux kernel to boot in normal text mode,
‘keep’ to preserve the graphics mode set using ‘GRUB_GFXMODE’,
‘widthxheight’[‘xdepth’] to set a particular
graphics mode, or a sequence of these separated by commas or semicolons to
try several modes in sequence. See gfxpayload.

Depending on your kernel, your distribution, your graphics card, and the
phase of the moon, note that using this option may cause GNU/Linux to suffer
from various display problems, particularly during the early part of the
boot sequence. If you have problems, set this option to ‘text’ and
GRUB will tell Linux to boot in normal text mode.

‘GRUB_DISABLE_OS_PROBER’

Normally, grub-mkconfig will try to use the external
os-prober program, if installed, to discover other operating
systems installed on the same system and generate appropriate menu entries
for them. Set this option to ‘true’ to disable this.

‘GRUB_OS_PROBER_SKIP_LIST’

List of space-separated FS UUIDs of filesystems to be ignored from os-prober
output. For efi chainloaders it’s <UUID>@<EFI FILE>

‘GRUB_DISABLE_SUBMENU’

Normally, grub-mkconfig will generate top level menu entry for
the kernel with highest version number and put all other found kernels
or alternative menu entries for recovery mode in submenu. For entries returned
by os-prober first entry will be put on top level and all others
in submenu. If this option is set to ‘y’, flat menu with all entries
on top level will be generated instead. Changing this option will require
changing existing values of ‘GRUB_DEFAULT’, ‘fallback’ (see fallback)
and ‘default’ (see default) environment variables as well as saved
default entry using grub-set-default and value used with
grub-reboot.

‘GRUB_ENABLE_CRYPTODISK’

If set to ‘y’, grub-mkconfig and grub-install will
check for encrypted disks and generate additional commands needed to access
them during boot. Note that in this case unattended boot is not possible
because GRUB will wait for passphrase to unlock encrypted container.

‘GRUB_INIT_TUNE’

Play a tune on the speaker when GRUB starts. This is particularly useful
for users unable to see the screen. The value of this option is passed
directly to play.

‘GRUB_BADRAM’

If this option is set, GRUB will issue a badram command to filter
out specified regions of RAM.

‘GRUB_PRELOAD_MODULES’

This option may be set to a list of GRUB module names separated by spaces.
Each module will be loaded as early as possible, at the start of
grub.cfg.

The following options are still accepted for compatibility with existing
configurations, but have better replacements:

‘GRUB_HIDDEN_TIMEOUT’

Wait this many seconds before displaying the menu. If ESC is pressed
during that time, display the menu and wait for input according to
‘GRUB_TIMEOUT’. If a hotkey associated with a menu entry is pressed,
boot the associated menu entry immediately. If the timeout expires before
either of these happens, display the menu for the number of seconds
specified in ‘GRUB_TIMEOUT’ before booting the default entry.

If you set ‘GRUB_HIDDEN_TIMEOUT’, you should also set
‘GRUB_TIMEOUT=0’ so that the menu is not displayed at all unless
ESC is pressed.

This option is unset by default, and is deprecated in favour of the less
confusing ‘GRUB_TIMEOUT_STYLE=countdown’ or
‘GRUB_TIMEOUT_STYLE=hidden’.

‘GRUB_HIDDEN_TIMEOUT_QUIET’

In conjunction with ‘GRUB_HIDDEN_TIMEOUT’, set this to ‘true’ to
suppress the verbose countdown while waiting for a key to be pressed before
displaying the menu.

This option is unset by default, and is deprecated in favour of the less
confusing ‘GRUB_TIMEOUT_STYLE=countdown’.

‘GRUB_HIDDEN_TIMEOUT_BUTTON’

Variant of ‘GRUB_HIDDEN_TIMEOUT’, used to support vendor-specific power
buttons. See Vendor power-on keys.

This option is unset by default, and is deprecated in favour of the less
confusing ‘GRUB_TIMEOUT_STYLE=countdown’ or
‘GRUB_TIMEOUT_STYLE=hidden’.

For more detailed customisation of grub-mkconfig’s output, you may
edit the scripts in /etc/grub.d directly.
/etc/grub.d/40_custom is particularly useful for adding entire custom
menu entries; simply type the menu entries you want to add at the end of
that file, making sure to leave at least the first two lines intact.

6.2 Writing full configuration files directly

grub.cfg is written in GRUB’s built-in scripting language, which has
a syntax quite similar to that of GNU Bash and other Bourne shell
derivatives.

Words

A word is a sequence of characters considered as a single unit by
GRUB. Words are separated by metacharacters, which are the following
plus space, tab, and newline:

{ } | & $ ; < >

Quoting may be used to include metacharacters in words; see below.

Reserved words

Reserved words have a special meaning to GRUB. The following words are
recognised as reserved when unquoted and either the first word of a simple
command or the third word of a for command:

! [[ ]] { }
case do done elif else esac fi for function
if in menuentry select then time until while

Not all of these reserved words have a useful purpose yet; some are reserved
for future expansion.

Quoting

Quoting is used to remove the special meaning of certain characters or
words. It can be used to treat metacharacters as part of a word, to prevent
reserved words from being recognised as such, and to prevent variable
expansion.

There are three quoting mechanisms: the escape character, single quotes, and
double quotes.

A non-quoted backslash (\) is the escape character. It preserves the
literal value of the next character that follows, with the exception of
newline.

Enclosing characters in single quotes preserves the literal value of each
character within the quotes. A single quote may not occur between single
quotes, even when preceded by a backslash.

Enclosing characters in double quotes preserves the literal value of all
characters within the quotes, with the exception of ‘$’ and ‘\’.
The ‘$’ character retains its special meaning within double quotes.
The backslash retains its special meaning only when followed by one of the
following characters: ‘$’, ‘"’, ‘\’, or newline. A
backslash-newline pair is treated as a line continuation (that is, it is
removed from the input stream and effectively ignored7). A double quote may be quoted within
double quotes by preceding it with a backslash.

Variable expansion

The ‘$’ character introduces variable expansion. The variable name to
be expanded may be enclosed in braces, which are optional but serve to
protect the variable to be expanded from characters immediately following it
which could be interpreted as part of the name.

Normal variable names begin with an alphabetic character, followed by zero
or more alphanumeric characters. These names refer to entries in the GRUB
environment (see Environment).

Positional variable names consist of one or more digits. They represent
parameters passed to function calls, with ‘$1’ representing the first
parameter, and so on.

The special variable name ‘?’ expands to the exit status of the most
recently executed command. When positional variable names are active, other
special variable names ‘@’, ‘*’ and ‘#’ are defined and they
expand to all positional parameters with necessary quoting, positional
parameters without any quoting, and positional parameter count respectively.

Comments

A word beginning with ‘#’ causes that word and all remaining characters
on that line to be ignored.

Simple commands

A simple command is a sequence of words separated by spaces or tabs
and terminated by a semicolon or a newline. The first word specifies the
command to be executed. The remaining words are passed as arguments to the
invoked command.

The return value of a simple command is its exit status. If the reserved
word ! precedes the command, then the return value is instead the
logical negation of the command’s exit status.

Compound commands

A compound command is one of the following:

for name in word …; do list; done

The list of words following in is expanded, generating a list of
items. The variable name is set to each element of this list in turn,
and list is executed each time. The return value is the exit status
of the last command that executes. If the expansion of the items following
in results in an empty list, no commands are executed, and the return
status is 0.

if list; then list; [elif list; then list;] … [else list;] fi

The iflist is executed. If its exit status is zero, the
thenlist is executed. Otherwise, each eliflist
is executed in turn, and if its exit status is zero, the corresponding
thenlist is executed and the command completes. Otherwise,
the elselist is executed, if present. The exit status is the
exit status of the last command executed, or zero if no condition tested
true.

while cond; do list; done

until cond; do list; done

The while command continuously executes the dolist as
long as the last command in cond returns an exit status of zero. The
until command is identical to the while command, except that
the test is negated; the dolist is executed as long as the
last command in cond returns a non-zero exit status. The exit status
of the while and until commands is the exit status of the last
dolist command executed, or zero if none was executed.

function name { command; … }

This defines a function named name. The body of the function is
the list of commands within braces, each of which must be terminated with a
semicolon or a newline. This list of commands will be executed whenever
name is specified as the name of a simple command. Function
definitions do not affect the exit status in $?. When executed, the
exit status of a function is the exit status of the last command executed in
the body.

Built-in Commands

Some built-in commands are also provided by GRUB script to help script
writers perform actions that are otherwise not possible. For example, these
include commands to jump out of a loop without fully completing it, etc.

break [n]

Exit from within a for, while, or until loop. If
n is specified, break n levels. n must be greater than
or equal to 1. If n is greater than the number of enclosing loops,
all enclosing loops are exited. The return value is 0 unless n is
not greater than or equal to 1.

continue [n]

Resume the next iteration of the enclosing for, while or
until loop. If n is specified, resume at the nth
enclosing loop. n must be greater than or equal to 1. If n
is greater than the number of enclosing loops, the last enclosing loop (the
top-level loop) is resumed. The return value is 0 unless n is
not greater than or equal to 1.

return [n]

Causes a function to exit with the return value specified by n. If
n is omitted, the return status is that of the last command executed
in the function body. If used outside a function the return status is
false.

setparams [arg] …

Replace positional parameters starting with $1 with arguments to
setparams.

shift [n]

The positional parameters from n+1 … are renamed to
$1…. Parameters represented by the numbers $# down to
$#-n+1 are unset. n must be a non-negative number less
than or equal to $#. If n is 0, no parameters are changed.
If n is not given, it is assumed to be 1. If n is greater
than $#, the positional parameters are not changed. The return
status is greater than zero if n is greater than $# or less
than zero; otherwise 0.

6.3 Multi-boot manual config

Currently autogenerating config files for multi-boot environments depends on
os-prober and has several shortcomings. While fixing it is scheduled for the
next release, meanwhile you can make use of the power of GRUB syntax and do it
yourself. A possible configuration is detailed here, feel free to adjust to your
needs.

First create a separate GRUB partition, big enough to hold GRUB. Some of the
following entries show how to load OS installer images from this same partition,
for that you obviously need to make the partition large enough to hold those
images as well.
Mount this partition on/mnt/boot and disable GRUB in all OSes and manually
install self-compiled latest GRUB with:

grub-install --boot-directory=/mnt/boot /dev/sda

In all the OSes install GRUB tools but disable installing GRUB in bootsector,
so you’ll have menu.lst and grub.cfg available for use. Also disable os-prober
use by setting:

Argument to search after –label is FS LABEL. You can also use UUIDs with –fs-uuid UUID instead of –label LABEL. You could also use direct root=hd0,msdosX but this is not recommended due to device name instability.

6.4 Embedding a configuration file into GRUB

GRUB supports embedding a configuration file directly into the core image,
so that it is loaded before entering normal mode. This is useful, for
example, when it is not straightforward to find the real configuration file,
or when you need to debug problems with loading that file.
grub-install uses this feature when it is not using BIOS disk
functions or when installing to a different disk from the one containing
/boot/grub, in which case it needs to use the search
command (see search) to find /boot/grub.

To embed a configuration file, use the -c option to
grub-mkimage. The file is copied into the core image, so it may
reside anywhere on the file system, and may be removed after running
grub-mkimage.

After the embedded configuration file (if any) is executed, GRUB will load
the ‘normal’ module (see normal), which will then read the real
configuration file from $prefix/grub.cfg. By this point, the
root variable will also have been set to the root device name. For
example, prefix might be set to ‘(hd0,1)/boot/grub’, and
root might be set to ‘hd0,1’. Thus, in most cases, the embedded
configuration file only needs to set the prefix and root
variables, and then drop through to GRUB’s normal processing. A typical
example of this might look like this:

(The ‘search_fs_uuid’ module must be included in the core image for this
example to work.)

In more complex cases, it may be useful to read other configuration files
directly from the embedded configuration file. This allows such things as
reading files not called grub.cfg, or reading files from a directory
other than that where GRUB’s loadable modules are installed. To do this,
include the ‘configfile’ and ‘normal’ modules in the core image,
and embed a configuration file that uses the configfile command to
load another file. The following example of this also requires the
echo, search_label, and test modules to be
included in the core image:

7 Theme file format

7.1 Introduction

The GRUB graphical menu supports themes that can customize the layout and
appearance of the GRUB boot menu. The theme is configured through a plain
text file that specifies the layout of the various GUI components (including
the boot menu, timeout progress bar, and text messages) as well as the
appearance using colors, fonts, and images. Example is available in docs/example_theme.txt

with “SVG 1.0 color names” (e.g., “cornflowerblue”) which must be specified in lowercase.

7.2.2 Fonts

The fonts GRUB uses “PFF2 font format” bitmap fonts. Fonts are specified
with full font names. Currently there is no
provision for a preference list of fonts, or deriving one font from another.
Fonts are loaded with the “loadfont” command in GRUB (loadfont). To see the list of
loaded fonts, execute the “lsfonts” command (lsfonts). If there are too many fonts to
fit on screen, do “set pager=1” before executing “lsfonts”.

7.2.3 Progress Bar

Figure 7.1

Figure 7.2

Progress bars are used to display the remaining time before GRUB boots the
default menu entry. To create a progress bar that will display the remaining
time before automatic boot, simply create a “progress_bar” component with
the id “__timeout__”. This indicates to GRUB that the progress bar should
be updated as time passes, and it should be made invisible if the countdown to
automatic boot is interrupted by the user.

Progress bars may optionally have text displayed on them. This text is
controlled by variable “text” which contains a printf template with the
only argument %d is the number of seconds remaining. Additionally special
values “@TIMEOUT_NOTIFICATION_SHORT@”, “@TIMEOUT_NOTIFICATION_MIDDLE@”,
“@TIMEOUT_NOTIFICATION_LONG@” are replaced with standard and translated
templates.

7.2.4 Circular Progress Indicator

The circular progress indicator functions similarly to the progress bar. When
given an id of “__timeout__”, GRUB updates the circular progress indicator’s
value to indicate the time remaining. For the circular progress indicator,
there are two images used to render it: the *center* image, and the *tick*
image. The center image is rendered in the center of the component, while the
tick image is used to render each mark along the circumference of the
indicator.

7.2.5 Labels

Text labels can be placed on the boot screen. The font, color, and horizontal
alignment can be specified for labels. If a label is given the id
“__timeout__”, then the “text” property for that label is also updated
with a message informing the user of the number of seconds remaining until
automatic boot. This is useful in case you want the text displayed somewhere
else instead of directly on the progress bar.

7.2.6 Boot Menu

The boot menu where GRUB displays the menu entries from the “grub.cfg” file.
It is a list of items, where each item has a title and an optional icon. The
icon is selected based on the *classes* specified for the menu entry. If
there is a PNG file named “myclass.png” in the “grub/themes/icons”
directory, it will be displayed for items which have the class *myclass*. The
boot menu can be customized in several ways, such as the font and color used
for the menu entry title, and by specifying styled boxes for the menu itself
and for the selected item highlight.

7.2.7 Styled Boxes

One of the most important features for customizing the layout is the use of
*styled boxes*. A styled box is composed of 9 rectangular (and potentially
empty) regions, which are used to seamlessly draw the styled box on screen:

Northwest (nw)

North (n)

Northeast (ne)

West (w)

Center (c)

East (e)

Southwest (sw)

South (s)

Southeast (se)

To support any size of box on screen, the center slice and the slices for the
top, bottom, and sides are all scaled to the correct size for the component on
screen, using the following rules:

The edge slices (north, south, east, and west) are scaled in the direction of the edge they are adjacent to. For instance, the west slice is scaled vertically.

The corner slices (northwest, northeast, southeast, and southwest) are not scaled.

The center slice is scaled to fill the remaining space in the middle.

As an example of how an image might be sliced up, consider the styled box
used for a terminal view.

Figure 7.3

7.2.8 Creating Styled Box Images

The Inkscape_ scalable vector graphics editor is a very useful tool for
creating styled box images. One process that works well for slicing a drawing
into the necessary image slices is:

Create or open the drawing you’d like use.

Create a new layer on the top of the layer stack. Make it visible. Select this layer as the current layer.

Draw 9 rectangles on your drawing where you’d like the slices to be. Clear the fill option, and set the stroke to 1 pixel wide solid stroke. The corners of the slices must meet precisely; if it is off by a single pixel, it will probably be evident when the styled box is rendered in the GRUB menu. You should probably go to File | Document Properties | Grids and enable a grid or create a guide (click on one of the rulers next to the drawing and drag over the drawing; release the mouse button to place the guide) to help place the rectangles precisely.

Right click on the center slice rectangle and choose Object Properties. Change the "Id" to “slice_c“ and click Set. Repeat this for the remaining 8 rectangles, giving them Id values of “slice_n“, “slice_ne“, “slice_e“, and so on according to the location.

Save the drawing.

Select all the slice rectangles. With the slice layer selected, you can simply press Ctrl+A to select all rectangles. The status bar should indicate that 9 rectangles are selected.

Click the layer hide icon for the slice layer in the layer palette. The rectangles will remain selected, even though they are hidden.

Choose File | Export Bitmap and check the *Batch export 9 selected objects* box. Make sure that *Hide all except selected* is unchecked. click *Export*. This will create PNG files in the same directory as the drawing, named after the slices. These can now be used for a styled box in a GRUB theme.

7.3 Theme File Manual

The theme file is a plain text file. Lines that begin with “#“ are ignored
and considered comments. (Note: This may not be the case if the previous line
ended where a value was expected.)

The theme file contains two types of statements:

Global properties.

Component construction.

7.3.1 Global Properties

7.3.2 Format

Global properties are specified with the simple format:

name1: value1

name2: "value which may contain spaces"

name3: #88F

In this example, name3 is assigned a color value.

7.3.3 Global Property List

title-text

Specifies the text to display at the top center of the screen as a title.

title-font

Defines the font used for the title message at the top of the screen.

title-color

Defines the color of the title message.

message-font

Currently unused. Left for backward compatibility.

message-color

Currently unused. Left for backward compatibility.

message-bg-color

Currently unused. Left for backward compatibility.

desktop-image

Specifies the image to use as the background. It will be scaled
to fit the screen size or proportionally scaled depending on the scale
method.

desktop-image-scale-method

Specifies the scaling method for the *desktop-image*. Options are
“stretch“, “crop“, “padding“, “fitwidth“, “fitheight“.
“stretch“ for fitting the screen size. Otherwise it is proportional
scaling of a part of *desktop-image* to the part of the screen.
“crop“ part of the *desktop-image* will be proportionally scaled to
fit the screen sizes. “padding“ the entire *desktop-image* will be
contained on the screen. “fitwidth“ for fitting the *desktop-image*’s
width with screen width. “fitheight“ for fitting the *desktop-image*’s
height with the screen height. Default is “stretch“.

desktop-image-h-align

Specifies the horizontal alignment of the *desktop-image* if
*desktop-image-scale-method* isn’t equeal to “stretch“. Options are
“left“, “center“, “right“. Default is “center“.

desktop-image-v-align

Specifies the vertical alignment of the *desktop-image* if
*desktop-image-scale-method* isn’t equeal to “stretch“. Options are
“top“, “center“, “bottom“. Default is “center“.

desktop-color

Specifies the color for the background if *desktop-image* is not
specified.

terminal-box

Specifies the file name pattern for the styled box slices used for the
command line terminal window. For example, “terminal-box: terminal_*.png“
will use the images “terminal_c.png“ as the center area, “terminal_n.png“
as the north (top) edge, “terminal_nw.png“ as the northwest (upper left)
corner, and so on. If the image for any slice is not found, it will simply
be left empty.

terminal-border

Specifies the border width of the terminal window.

terminal-left

Specifies the left coordinate of the terminal window.

terminal-top

Specifies the top coordinate of the terminal window.

terminal-width

Specifies the width of the terminal window.

terminal-height

Specifies the height of the terminal window.

7.3.4 Component Construction

Greater customizability comes is provided by components. A tree of components
forms the user interface. *Containers* are components that can contain other
components, and there is always a single root component which is an instance
of a *canvas* container.

Components are created in the theme file by prefixing the type of component
with a ’+’ sign:

+ label { text="GRUB" font="aqui 11" color="#8FF" }

properties of a component are specified as "name = value" (whitespace
surrounding tokens is optional and is ignored) where *value* may be:

a single word (e.g., “align = center“, “color = #FF8080“),

a quoted string (e.g., “text = "Hello, World!"“), or

a tuple (e.g., “preferred_size = (120, 80)“).

7.3.5 Component List

The following is a list of the components and the properties they support.

label
A label displays a line of text.

Properties:

id

Set to “__timeout__“ to display the time elapsed to an automatical
boot of the default entry.

text

The text to display. If “id“ is set to “__timeout__“ and no
“text“ property is set then the amount of seconds will be shown.
If set to “@KEYMAP_SHORT@“, “@KEYMAP_MIDDLE@“ or
“@KEYMAP_LONG@“ then predefined hotkey information will be shown.

font

The font to use for text display.

color

The color of the text.

align

The horizontal alignment of the text within the component.
Options are “left“, “center“ and “right“.

visible

Set to “false“ to hide the label.

image
A component that displays an image. The image is scaled to fit
the component.

Properties:

file

The full path to the image file to load.

progress_bar
Displays a horizontally oriented progress bar. It can be rendered using
simple solid filled rectangles, or using a pair of pixmap styled boxes.

Properties:

id

Set to “__timeout__“ to display the time elapsed to an automatical
boot of the default entry.

fg_color

The foreground color for plain solid color rendering.

bg_color

The background color for plain solid color rendering.

border_color

The border color for plain solid color rendering.

text_color

The text color.

bar_style

The styled box specification for the frame of the progress bar.
Example: “progress_frame_*.png“
If the value is equal to “highlight_style“ then no styled boxes
will be shown.

highlight_style

The styled box specification for the highlighted region of the
progress bar. This box will be used to paint just the highlighted region
of the bar, and will be increased in size as the bar nears completion.
Example: “progress_hl_*.png“.
If the value is equal to “bar_style“ then no styled boxes
will be shown.

highlight_overlay

If this option is set to “true“ then the highlight box
side slices (every slice except the center slice) will overlay the
frame box side slices. And the center slice of the highlight box
can move all the way (from top to bottom), being drawn on the center
slice of the frame box. That way we can make a progress bar with
round-shaped edges so there won’t be a free space from the highlight to
the frame in top and bottom scrollbar positions. Default is “false“.

font

The font to use for progress bar.

text

The text to display on the progress bar. If the progress bar’s ID
is set to “__timeout__“ and the value of this property is set to
“@TIMEOUT_NOTIFICATION_SHORT@“, “@TIMEOUT_NOTIFICATION_MIDDLE@“
or “@TIMEOUT_NOTIFICATION_LONG@“, then GRUB will update this
property with an informative message as the timeout approaches.

circular_progress
Displays a circular progress indicator. The appearance of this component
is determined by two images: the *center* image and the *tick* image. The
center image is generally larger and will be drawn in the center of the
component. Around the circumference of a circle within the component, the
tick image will be drawn a certain number of times, depending on the
properties of the component.

Properties:

id

Set to “__timeout__“ to display the time elapsed to an automatical
boot of the default entry.

The image file pattern for the scroll bar thumb (the part of the scroll
bar that moves as scrolling occurs).
Example: “scrollbar_thumb_*.png“

scrollbar_thumb_overlay

If this option is set to “true“ then the scrollbar thumb
side slices (every slice except the center slice) will overlay the
scrollbar frame side slices. And the center slice of the scrollbar_thumb
can move all the way (from top to bottom), being drawn on the center
slice of the scrollbar frame. That way we can make a scrollbar with
round-shaped edges so there won’t be a free space from the thumb to
the frame in top and bottom scrollbar positions. Default is “false“.

scrollbar_slice

The menu frame styled box’s slice in which the scrollbar will be
drawn. Possible values are “west“, “center“, “east“ (default).
“west“ - the scrollbar will be drawn in the west slice (right-aligned).
“east“ - the scrollbar will be drawn in the east slice (left-aligned).
“center“ - the scrollbar will be drawn in the center slice.
Note: in case of “center“ slice:
a) If the scrollbar should be drawn then boot menu entry’s width is
decreased by the scrollbar’s width and the scrollbar is drawn at the
right side of the center slice.
b) If the scrollbar won’t be drawn then the boot menu entry’s width
is the width of the center slice.
c) We don’t necessary need the menu pixmap box to display the scrollbar.

scrollbar_left_pad

The left scrollbar padding in pixels.
Unused if “scrollbar_slice“ is “west“.

scrollbar_right_pad

The right scrollbar padding in pixels.
Unused if “scrollbar_slice“ is “east“.

scrollbar_top_pad

The top scrollbar padding in pixels.

scrollbar_bottom_pad

The bottom scrollbar padding in pixels.

visible

Set to “false“ to hide the boot menu.

canvas
Canvas is a container that allows manual placement of components within it.
It does not alter the positions of its child components. It assigns all
child components their preferred sizes.

hbox
The *hbox* container lays out its children from left to right, giving each
one its preferred width. The height of each child is set to the maximum of
the preferred heights of all children.

vbox
The *vbox* container lays out its children from top to bottom, giving each
one its preferred height. The width of each child is set to the maximum of
the preferred widths of all children.

7.3.6 Common properties

The following properties are supported by all components:

‘left’

The distance from the left border of container to left border of the object in either of three formats:

x

Value in pixels

p%

Percentage

p%+x

mixture of both

‘top’

The distance from the left border of container to left border of the object in same format.

‘width’

The width of object in same format.

‘height’

The height of object in same format.

‘id’

The identifier for the component. This can be any arbitrary string.
The ID can be used by scripts to refer to various components in the GUI
component tree. Currently, there is one special ID value that GRUB
recognizes:

“__timeout__“

Component with this ID will be updated by GRUB and will indicate
time elapsed to an automatical boot of the default entry.
Affected components: “label“, “circular_progress“, “progress_bar“.

Then follow instructions printed out by grub-mknetdir on configuring your DHCP
server.

After GRUB has started, files on the TFTP server will be accessible via the
‘(tftp)’ device.

The server IP address can be controlled by changing the
‘(tftp)’ device name to ‘(tftp,server-ip)’. Note that
this should be changed both in the prefix and in any references to the
device name in the configuration file.

GRUB provides several environment variables which may be used to inspect or
change the behaviour of the PXE device. In the following description
<interface> is placeholder for the name of network interface (platform
dependent):

9 Using GRUB via a serial line

This chapter describes how to use the serial terminal support in GRUB.

If you have many computers or computers with no display/keyboard, it
could be very useful to control the computers through serial
communications. To connect one computer with another via a serial line,
you need to prepare a null-modem (cross) serial cable, and you may need
to have multiport serial boards, if your computer doesn’t have extra
serial ports. In addition, a terminal emulator is also required, such as
minicom. Refer to a manual of your operating system, for more
information.

As for GRUB, the instruction to set up a serial terminal is quite
simple. Here is an example:

The command serial initializes the serial unit 0 with the
speed 9600bps. The serial unit 0 is usually called ‘COM1’, so, if
you want to use COM2, you must specify ‘--unit=1’ instead. This
command accepts many other options, so please refer to serial,
for more details.

The commands terminal_input (see terminal_input) and
terminal_output (see terminal_output) choose which type of
terminal you want to use. In the case above, the terminal will be a
serial terminal, but you can also pass console to the command,
as ‘terminal_input serial console’. In this case, a terminal in which
you press any key will be selected as a GRUB terminal. In the example above,
note that you need to put both commands on the same command line, as you
will lose the ability to type commands on the console after the first
command.

However, note that GRUB assumes that your terminal emulator is
compatible with VT100 by default. This is true for most terminal
emulators nowadays, but you should pass the option --dumb to
the command if your terminal emulator is not VT100-compatible or
implements few VT100 escape sequences. If you specify this option then
GRUB provides you with an alternative menu interface, because the normal
menu requires several fancy features of your terminal.

10 Using GRUB with vendor power-on keys

Some laptop vendors provide an additional power-on button which boots
another OS. GRUB supports such buttons with the ‘GRUB_TIMEOUT_BUTTON’,
‘GRUB_TIMEOUT_STYLE_BUTTON’, ‘GRUB_DEFAULT_BUTTON’, and
‘GRUB_BUTTON_CMOS_ADDRESS’ variables in default/grub (see Simple configuration). ‘GRUB_TIMEOUT_BUTTON’,
‘GRUB_TIMEOUT_STYLE_BUTTON’, and ‘GRUB_DEFAULT_BUTTON’ are used
instead of the corresponding variables without the ‘_BUTTON’ suffix
when powered on using the special button. ‘GRUB_BUTTON_CMOS_ADDRESS’
is vendor-specific and partially model-specific. Values known to the GRUB
team are:

11 GRUB image files

GRUB consists of several images: a variety of bootstrap images for starting
GRUB in various ways, a kernel image, and a set of modules which are
combined with the kernel image to form a core image. Here is a short
overview of them.

boot.img

On PC BIOS systems, this image is the first part of GRUB to start. It is
written to a master boot record (MBR) or to the boot sector of a partition.
Because a PC boot sector is 512 bytes, the size of this image is exactly 512
bytes.

The sole function of boot.img is to read the first sector of the core
image from a local disk and jump to it. Because of the size restriction,
boot.img cannot understand any file system structure, so
grub-install hardcodes the location of the first sector of the
core image into boot.img when installing GRUB.

diskboot.img

This image is used as the first sector of the core image when booting from a
hard disk. It reads the rest of the core image into memory and starts the
kernel. Since file system handling is not yet available, it encodes the
location of the core image using a block list format.

cdboot.img

This image is used as the first sector of the core image when booting from a
CD-ROM drive. It performs a similar function to diskboot.img.

pxeboot.img

This image is used as the start of the core image when booting from the
network using PXE. See Network.

lnxboot.img

This image may be placed at the start of the core image in order to make
GRUB look enough like a Linux kernel that it can be booted by LILO using an
‘image=’ section.

kernel.img

This image contains GRUB’s basic run-time facilities: frameworks for device
and file handling, environment variables, the rescue mode command-line
parser, and so on. It is rarely used directly, but is built into all core
images.

core.img

This is the core image of GRUB. It is built dynamically from the kernel
image and an arbitrary list of modules by the grub-mkimage
program. Usually, it contains enough modules to access /boot/grub,
and loads everything else (including menu handling, the ability to load
target operating systems, and so on) from the file system at run-time. The
modular design allows the core image to be kept small, since the areas of
disk where it must be installed are often as small as 32KB.

See BIOS installation, for details on where the core image can be
installed on PC systems.

*.mod

Everything else in GRUB resides in dynamically loadable modules. These are
often loaded automatically, or built into the core image if they are
essential, but may also be loaded manually using the insmod
command (see insmod).

For GRUB Legacy users

GRUB 2 has a different design from GRUB Legacy, and so correspondences with
the images it used cannot be exact. Nevertheless, GRUB Legacy users often
ask questions in the terms they are familiar with, and so here is a brief
guide to how GRUB 2’s images relate to that.

stage1

Stage 1 from GRUB Legacy was very similar to boot.img in GRUB 2, and
they serve the same function.

*_stage1_5

In GRUB Legacy, Stage 1.5’s function was to include enough filesystem code
to allow the much larger Stage 2 to be read from an ordinary filesystem. In
this respect, its function was similar to core.img in GRUB 2.
However, core.img is much more capable than Stage 1.5 was; since it
offers a rescue shell, it is sometimes possible to recover manually in the
event that it is unable to load any other modules, for example if partition
numbers have changed. core.img is built in a more flexible way,
allowing GRUB 2 to support reading modules from advanced disk types such as
LVM and RAID.

GRUB Legacy could run with only Stage 1 and Stage 2 in some limited
configurations, while GRUB 2 requires core.img and cannot work
without it.

stage2

GRUB 2 has no single Stage 2 image. Instead, it loads modules from
/boot/grub at run-time.

stage2_eltorito

In GRUB 2, images for booting from CD-ROM drives are now constructed using
cdboot.img and core.img, making sure that the core image
contains the ‘iso9660’ module. It is usually best to use the
grub-mkrescue program for this.

nbgrub

There is as yet no equivalent for nbgrub in GRUB 2; it was used by
Etherboot and some other network boot loaders.

pxegrub

In GRUB 2, images for PXE network booting are now constructed using
pxeboot.img and core.img, making sure that the core image
contains the ‘pxe’ and ‘pxecmd’ modules. See Network.

13 Filesystem syntax and semantics

GRUB uses a special syntax for specifying disk drives which can be
accessed by BIOS. Because of BIOS limitations, GRUB cannot distinguish
between IDE, ESDI, SCSI, or others. You must know yourself which BIOS
device is equivalent to which OS device. Normally, that will be clear if
you see the files in a device or use the command search
(see search).

13.1 How to specify devices

The device syntax is like this:

(device[,partmap-name1part-num1[,partmap-name2part-num2[,...]]])

‘[]’ means the parameter is optional. device depends on the disk
driver in use. BIOS and EFI disks use either ‘fd’ or ‘hd’ followed
by a digit, like ‘fd0’, or ‘cd’.
AHCI, PATA (ata), crypto, USB use the name of driver followed by a number.
Memdisk and host are limited to one disk and so it’s refered just by driver
name.
RAID (md), ofdisk (ieee1275 and nand), LVM (lvm), LDM, virtio (vdsk)
and arcdisk (arc) use intrinsic name of disk prefixed by driver name.
Additionally just “nand” refers to the disk aliased as “nand”.
Conflicts are solved by suffixing a number if necessarry.
Commas need to be escaped.
Loopback uses whatever name specified to loopback command.
Hostdisk uses names specified in device.map as long as it’s of the form
[fhc]d[0-9]* or hostdisk/<OS DEVICE>.
For crypto and RAID (md) additionally you can use the syntax
<driver name>uuid/<uuid>. For LVM additionally you can use the syntax
lvmid/<volume-group-uuid>/<volume-uuid>.

part-num represents the partition number of device, starting
from one. partname is optional but is recommended since disk may have
several top-level partmaps. Specifying third and later component you can access
to subpartitions.

The syntax ‘(hd0)’ represents using the entire disk (or the
MBR when installing GRUB), while the syntax ‘(hd0,1)’
represents using the first partition of the disk (or the boot sector
of the partition when installing GRUB).

If you enabled the network support, the special drives
(protocol[,server]) are also available. Supported protocols
are ‘http’ and ‘tftp’. If server is omitted, value of
environment variable ‘net_default_server’ is used.
Before using the network drive, you must initialize the network.
See Network, for more information.

13.2 How to specify files

There are two ways to specify files, by absolute file name and by
block list.

An absolute file name resembles a Unix absolute file name, using
‘/’ for the directory separator (not ‘\’ as in DOS). One
example is ‘(hd0,1)/boot/grub/grub.cfg’. This means the file
/boot/grub/grub.cfg in the first partition of the first hard
disk. If you omit the device name in an absolute file name, GRUB uses
GRUB’s root device implicitly. So if you set the root device to,
say, ‘(hd1,1)’ by the command ‘set root=(hd1,1)’ (see set),
then /boot/kernel is the same as (hd1,1)/boot/kernel.

On ZFS filesystem the first path component must be
volume‘@’[snapshot].
So ‘/rootvol@snap-129/boot/grub/grub.cfg’ refers to file
‘/boot/grub/grub.cfg’ in snapshot of volume ‘rootvol’ with name
‘snap-129’. Trailing ‘@’ after volume name is mandatory even if
snapshot name is omitted.

14 GRUB’s user interface

GRUB has both a simple menu interface for choosing preset entries from a
configuration file, and a highly flexible command-line for performing
any desired combination of boot commands.

GRUB looks for its configuration file as soon as it is loaded. If one
is found, then the full menu interface is activated using whatever
entries were found in the file. If you choose the command-line menu
option, or if the configuration file was not found, then GRUB drops to
the command-line interface.

14.1 The flexible command-line interface

The command-line interface provides a prompt and after it an editable
text area much like a command-line in Unix or DOS. Each command is
immediately executed after it is entered8. The commands (see Command-line and menu entry commands) are a
subset of those available in the configuration file, used with exactly
the same syntax.

Cursor movement and editing of the text on the line can be done via a
subset of the functions available in the Bash shell:

C-f

PC right key

Move forward one character.

C-b

PC left key

Move back one character.

C-a

HOME

Move to the start of the line.

C-e

END

Move the the end of the line.

C-d

DEL

Delete the character underneath the cursor.

C-h

BS

Delete the character to the left of the cursor.

C-k

Kill the text from the current cursor position to the end of the line.

C-u

Kill backward from the cursor to the beginning of the line.

C-y

Yank the killed text back into the buffer at the cursor.

C-p

PC up key

Move up through the history list.

C-n

PC down key

Move down through the history list.

When typing commands interactively, if the cursor is within or before
the first word in the command-line, pressing the TAB key (or
C-i) will display a listing of the available commands, and if the
cursor is after the first word, the TAB will provide a
completion listing of disks, partitions, and file names depending on the
context. Note that to obtain a list of drives, one must open a
parenthesis, as root (.

Note that you cannot use the completion functionality in the TFTP
filesystem. This is because TFTP doesn’t support file name listing for
the security.

14.2 The simple menu interface

The menu interface is quite easy to use. Its commands are both
reasonably intuitive and described on screen.

Basically, the menu interface provides a list of boot entries to
the user to choose from. Use the arrow keys to select the entry of
choice, then press RET to run it. An optional timeout is
available to boot the default entry (the first one if not set), which is
aborted by pressing any key.

Commands are available to enter a bare command-line by pressing c
(which operates exactly like the non-config-file version of GRUB, but
allows one to return to the menu if desired by pressing ESC) or to
edit any of the boot entries by pressing e.

If you protect the menu interface with a password (see Security),
all you can do is choose an entry by pressing RET, or press
p to enter the password.

15 GRUB environment variables

GRUB supports environment variables which are rather like those offered by
all Unix-like systems. Environment variables have a name, which is unique
and is usually a short identifier, and a value, which is an arbitrary string
of characters. They may be set (see set), unset (see unset), or
looked up (see Shell-like scripting) by name.

A number of environment variables have special meanings to various parts of
GRUB. Others may be used freely in GRUB configuration files.

15.1.1 biosnum

When chain-loading another boot loader (see Chain-loading), GRUB may
need to know what BIOS drive number corresponds to the root device
(see root) so that it can set up registers properly. If the
biosnum variable is set, it overrides GRUB’s own means of guessing
this.

For an alternative approach which also changes BIOS drive mappings for the
chain-loaded system, see drivemap.

15.1.4 cmdpath

The location from which core.img was loaded as an absolute
directory name (see File name syntax). This is set by GRUB at
startup based on information returned by platform firmware. Not every
platform provides this information and some may return only device
without path name.

15.1.5 color_highlight

This variable contains the “highlight” foreground and background terminal
colors, separated by a slash (‘/’). Setting this variable changes
those colors. For the available color names, see color_normal.

15.1.6 color_normal

This variable contains the “normal” foreground and background terminal
colors, separated by a slash (‘/’). Setting this variable changes
those colors. Each color must be a name from the following list:

black

blue

green

cyan

red

magenta

brown

light-gray

dark-gray

light-blue

light-green

light-cyan

light-red

light-magenta

yellow

white

The default is ‘light-gray/black’.

The color support support varies from terminal to terminal.

‘morse’ has no color support at all.

‘mda_text’ color support is limited to highlighting by
black/white reversal.

‘console’ on ARC, EMU and IEEE1275, ‘serial_*’ and
‘spkmodem’ are governed by terminfo and support
only 8 colors if in modes ‘vt100-color’ (default for console on emu),
‘arc’ (default for console on ARC), ‘ieee1275’ (default
for console on IEEE1275). When in mode ‘vt100’
then the color support is limited to highlighting by black/white
reversal. When in mode ‘dumb’ there is no color support.

When console supports no colors this setting is ignored.
When console supports 8 colors, then the colors from the
second half of the previous list are mapped to the
matching colors of first half.

‘console’ on EFI and BIOS and ‘vga_text’ support all 16 colors.

‘gfxterm’ supports all 16 colors and would be theoretically extendable
to support whole rgb24 palette but currently there is no compelling reason
to go beyond the current 16 colors.

15.1.7 debug

This variable may be set to enable debugging output from various components
of GRUB. The value is a list of debug facility names separated by
whitespace or ‘,’, or ‘all’ to enable all available debugging
output. The facility names are the first argument to grub_dprintf. Consult
source for more details.

If the entry is in a submenu, then it must be identified using the titles of
each of the submenus starting from the top level followed by the number or
title of the menu entry itself, separated by ‘>’. For example, take
the following menu structure:

15.1.10 gfxmode

If this variable is set, it sets the resolution used on the ‘gfxterm’
graphical terminal. Note that you can only use modes which your graphics
card supports via VESA BIOS Extensions (VBE), so for example native LCD
panel resolutions may not be available. The default is ‘auto’, which
selects a platform-specific default that should look reasonable. Supported
modes can be listed by ‘videoinfo’ command in GRUB.

The resolution may be specified as a sequence of one or more modes,
separated by commas (‘,’) or semicolons (‘;’); each will be tried
in turn until one is found. Each mode should be either ‘auto’,
‘widthxheight’, or
‘widthxheightxdepth’.

15.1.11 gfxpayload

If this variable is set, it controls the video mode in which the Linux
kernel starts up, replacing the ‘vga=’ boot option (see linux). It
may be set to ‘text’ to force the Linux kernel to boot in normal text
mode, ‘keep’ to preserve the graphics mode set using ‘gfxmode’, or
any of the permitted values for ‘gfxmode’ to set a particular graphics
mode (see gfxmode).

Depending on your kernel, your distribution, your graphics card, and the
phase of the moon, note that using this option may cause GNU/Linux to suffer
from various display problems, particularly during the early part of the
boot sequence. If you have problems, set this variable to ‘text’ and
GRUB will tell Linux to boot in normal text mode.

The default is platform-specific. On platforms with a native text mode
(such as PC BIOS platforms), the default is ‘text’. Otherwise the
default may be ‘auto’ or a specific video mode.

15.1.18 menu_color_highlight

This variable contains the foreground and background colors to be used for
the highlighted menu entry, separated by a slash (‘/’). Setting this
variable changes those colors. For the available color names,
see color_normal.

15.1.19 menu_color_normal

This variable contains the foreground and background colors to be used for
non-highlighted menu entries, separated by a slash (‘/’). Setting this
variable changes those colors. For the available color names,
see color_normal.

15.1.32 pager

15.1.33 prefix

The location of the ‘/boot/grub’ directory as an absolute file name
(see File name syntax). This is normally set by GRUB at startup based
on information provided by grub-install. GRUB modules are
dynamically loaded from this directory, so it must be set correctly in order
for many parts of GRUB to work.

15.1.36 pxe_default_server

15.1.37 root

The root device name (see Device syntax). Any file names that do not
specify an explicit device name are read from this device. The default is
normally set by GRUB at startup based on the value of ‘prefix’
(see prefix).

For example, if GRUB was installed to the first partition of the first hard
disk, then ‘prefix’ might be set to ‘(hd0,msdos1)/boot/grub’ and
‘root’ to ‘hd0,msdos1’.

15.1.40 timeout

If this variable is set, it specifies the time in seconds to wait for
keyboard input before booting the default menu entry. A timeout of ‘0’
means to boot the default entry immediately without displaying the menu; a
timeout of ‘-1’ (or unset) means to wait indefinitely.

If ‘timeout_style’ (see timeout_style) is set to ‘countdown’
or ‘hidden’, the timeout is instead counted before the menu is
displayed.

15.1.41 timeout_style

This variable may be set to ‘menu’, ‘countdown’, or ‘hidden’
to control the way in which the timeout (see timeout) interacts with
displaying the menu. See the documentation of ‘GRUB_TIMEOUT_STYLE’
(see Simple configuration) for details.

15.2 The GRUB environment block

It is often useful to be able to remember a small amount of information from
one boot to the next. For example, you might want to set the default menu
entry based on what was selected the last time. GRUB deliberately does not
implement support for writing files in order to minimise the possibility of
the boot loader being responsible for file system corruption, so a GRUB
configuration file cannot just create a file in the ordinary way. However,
GRUB provides an “environment block” which can be used to save a small
amount of state.

The environment block is a preallocated 1024-byte file, which normally lives
in /boot/grub/grubenv (although you should not assume this). At boot
time, the load_env command (see load_env) loads environment
variables from it, and the save_env (see save_env) command
saves environment variables to it. From a running system, the
grub-editenv utility can be used to edit the environment block.

For safety reasons, this storage is only available when installed on a plain
disk (no LVM or RAID), using a non-checksumming filesystem (no ZFS), and
using BIOS or EFI functions (no ATA, USB or IEEE1275).

16 The list of available commands

In this chapter, we list all commands that are available in GRUB.

Commands belong to different groups. A few can only be used in
the global section of the configuration file (or “menu”); most
of them can be entered on the command-line and can be used either
anywhere in the menu or specifically in the menu entries.

16.1.1 menuentry

This defines a GRUB menu entry named title. When this entry is
selected from the menu, GRUB will set the chosen environment variable
to value of --id if --id is given, execute the list of
commands given within braces, and if the last command in the list returned
successfully and a kernel was loaded it will execute the boot command.

The --class option may be used any number of times to group menu
entries into classes. Menu themes may display different classes using
different styles.

16.2.1 serial

Initialize a serial device. unit is a number in the range 0-3
specifying which serial port to use; default is 0, which corresponds to
the port often called COM1. port is the I/O port where the UART
is to be found; if specified it takes precedence over unit.
speed is the transmission speed; default is 9600. word and
stop are the number of data bits and stop bits. Data bits must
be in the range 5-8 and stop bits must be 1 or 2. Default is 8 data
bits and one stop bit. parity is one of ‘no’, ‘odd’,
‘even’ and defaults to ‘no’.

The serial port is not used as a communication channel unless the
terminal_input or terminal_output command is used
(see terminal_input, see terminal_output).

16.2.4 terminfo

Define the capabilities of your terminal by giving the name of an entry in
the terminfo database, which should correspond roughly to a ‘TERM’
environment variable in Unix.

The currently available terminal types are ‘vt100’, ‘vt100-color’,
‘ieee1275’, and ‘dumb’. If you need other terminal types, please
contact us to discuss the best way to include support for these in GRUB.

The -a (--ascii), -u (--utf8), and
-v (--visual-utf8) options control how non-ASCII text is
displayed. -a specifies an ASCII-only terminal; -u
specifies logically-ordered UTF-8; and -v specifies
"visually-ordered UTF-8" (in other words, arranged such that a terminal
emulator without bidirectional text support will display right-to-left text
in the proper order; this is not really proper UTF-8, but a workaround).

If no option or terminal type is specified, the current terminal type is
printed.

16.3.2 acpi

Modern BIOS systems normally implement the Advanced Configuration and Power
Interface (ACPI), and define various tables that describe the interface
between an ACPI-compliant operating system and the firmware. In some cases,
the tables provided by default only work well with certain operating
systems, and it may be necessary to replace some of them.

Normally, this command will replace the Root System Description Pointer
(RSDP) in the Extended BIOS Data Area to point to the new tables. If the
--no-ebda option is used, the new tables will be known only to
GRUB, but may be used by GRUB’s EFI emulation.

16.3.5 background_image

Command: background_image[[--mode ‘stretch’|‘normal’] file]

Load background image for active terminal from file. Image is stretched
to fill up entire screen unless option --mode ‘normal’ is given.
Without arguments remove currently loaded background image. Background image
can be changed only when using ‘gfxterm’ for terminal output.

16.3.6 badram

Command: badramaddr,mask[,addr,mask...]

Filter out bad RAM.

This command notifies the memory manager that specified regions of
RAM ought to be filtered out (usually, because they’re damaged). This
remains in effect after a payload kernel has been loaded by GRUB, as
long as the loaded kernel obtains its memory map from GRUB. Kernels that
support this include Linux, GNU Mach, the kernel of FreeBSD and Multiboot
kernels in general.

Syntax is the same as provided by the Memtest86+ utility: a list of address/mask pairs. Given a page-aligned
address and a base address / mask pair, if all the bits of the page-aligned
address that are enabled by the mask match with the base address, it means
this page is to be filtered. This syntax makes it easy to represent patterns
that are often result of memory damage, due to physical distribution of memory
cells.

16.3.9 cat

Command: cat[--dos] file

Display the contents of the file file. This command may be useful
to remind you of your OS’s root partition:

grub> cat /etc/fstab

If the --dos option is used, then carriage return / new line pairs
will be displayed as a simple new line. Otherwise, the carriage return will
be displayed as a control character (‘<d>’) to make it easier to see
when boot problems are caused by a file formatted using DOS-style line
endings.

16.3.10 chainloader

Command: chainloader[--force] file

Load file as a chain-loader. Like any other file loaded by the
filesystem code, it can use the blocklist notation (see Block list syntax) to grab the first sector of the current partition with ‘+1’.
If you specify the option --force, then load file forcibly,
whether it has a correct signature or not. This is required when you want to
load a defective boot loader, such as SCO UnixWare 7.1.

16.3.16 configfile

Command: configfilefile

Load file as a configuration file. If file defines any menu
entries, then show a menu containing them immediately. Any environment
variable changes made by the commands in file will not be preserved
after configfile returns.

16.3.20 date

Command: date[[year-]month-day] [hour:minute[:second]]

With no arguments, print the current date and time.

Otherwise, take the current date and time, change any elements specified as
arguments, and set the result as the new date and time. For example, ‘date
01-01’ will set the current month and day to January 1, but leave the year,
hour, minute, and second unchanged.

16.3.22 distrust

Command: distrustpubkey_id

Remove public key pubkey_id from GRUB’s keyring of trusted keys.
pubkey_id is the last four bytes (eight hexadecimal digits) of
the GPG v4 key id, which is also the output of list_trusted
(see list_trusted). Outside of GRUB, the key id can be obtained
using gpg --fingerprint).
These keys are used to validate signatures when environment variable
check_signatures is set to enforce
(see check_signatures), and by some invocations of
verify_detached (see verify_detached). See Using digital signatures, for more information.

16.3.23 drivemap

Command: drivemap-l|-r|[-s] from_drive to_drive

Without options, map the drive from_drive to the drive to_drive.
This is necessary when you chain-load some operating systems, such as DOS,
if such an OS resides at a non-first drive. For convenience, any partition
suffix on the drive is ignored, so you can safely use ${root} as a
drive specification.

With the -s option, perform the reverse mapping as well, swapping
the two drives.

16.3.24 echo

Command: echo[-n] [-e] string …

Display the requested text and, unless the -n option is used, a
trailing new line. If there is more than one string, they are separated by
spaces in the output. As usual in GRUB commands, variables may be
substituted using ‘${var}’.

The -e option enables interpretation of backslash escapes. The
following sequences are recognised:

\\

backslash

\a

alert (BEL)

\c

suppress trailing new line

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

When interpreting backslash escapes, backslash followed by any other
character will print that character.

16.3.29 gptsync

Command: gptsyncdevice [partition[+/-[type]]] …

Disks using the GUID Partition Table (GPT) also have a legacy Master Boot
Record (MBR) partition table for compatibility with the BIOS and with older
operating systems. The legacy MBR can only represent a limited subset of
GPT partition entries.

This command populates the legacy MBR with the specified partition
entries on device. Up to three partitions may be used.

type is an MBR partition type code; prefix with ‘0x’ if you want
to enter this in hexadecimal. The separator between partition and
type may be ‘+’ to make the partition active, or ‘-’ to make
it inactive; only one partition may be active. If both the separator and
type are omitted, then the partition will be inactive.

When list of files is given, hash of each file is computed and printed,
followed by file name, each file on a new line.

When option --check is given, it points to a file that contains
list of hash name pairs in the same format as used by UNIX
md5sum command. Option --prefix
may be used to give directory where files are located. Hash verification
stops after the first mismatch was found unless option --keep-going
was given. The exit code $? is set to 0 if hash verification
is successful. If it fails, $? is set to a nonzero value.

16.3.33 initrd

Command: initrdfile

Load an initial ramdisk for a Linux kernel image, and set the appropriate
parameters in the Linux setup area in memory. This may only be used after
the linux command (see linux) has been run. See also
GNU/Linux.

16.3.34 initrd16

Command: initrd16file

Load an initial ramdisk for a Linux kernel image to be booted in 16-bit
mode, and set the appropriate parameters in the Linux setup area in memory.
This may only be used after the linux16 command (see linux16)
has been run. See also GNU/Linux.

16.3.35 insmod

16.3.36 keystatus

Command: keystatus[--shift] [--ctrl] [--alt]

Return true if the Shift, Control, or Alt modifier keys are held down, as
requested by options. This is useful in scripting, to allow some user
control over behaviour without having to wait for a keypress.

Checking key modifier status is only supported on some platforms. If invoked
without any options, the keystatus command returns true if and
only if checking key modifier status is supported.

16.3.37 linux

Command: linuxfile …

Load a Linux kernel image from file. The rest of the line is passed
verbatim as the kernel command-line. Any initrd must be reloaded
after using this command (see initrd).

On x86 systems, the kernel will be booted using the 32-bit boot protocol.
Note that this means that the ‘vga=’ boot option will not work; if you
want to set a special video mode, you will need to use GRUB commands such as
‘set gfxpayload=1024x768’ or ‘set gfxpayload=keep’ (to keep the
same mode as used in GRUB) instead. GRUB can automatically detect some uses
of ‘vga=’ and translate them to appropriate settings of
‘gfxpayload’. The linux16 command (see linux16) avoids
this restriction.

16.3.38 linux16

Load a Linux kernel image from file in 16-bit mode. The rest of the
line is passed verbatim as the kernel command-line. Any initrd must
be reloaded after using this command (see initrd16).

The kernel will be booted using the traditional 16-bit boot protocol. As
well as bypassing problems with ‘vga=’ described in linux, this
permits booting some other programs that implement the Linux boot protocol
for the sake of convenience.

16.3.40 list_trusted

Command: list_trusted

List all public keys trusted by GRUB for validating signatures.
The output is in GPG’s v4 key fingerprint format (i.e., the output of
gpg --fingerprint). The least significant four bytes (last
eight hexadecimal digits) can be used as an argument to
distrust (see distrust).
See Using digital signatures, for more information about uses for
these keys.

16.3.41 load_env

Load all variables from the environment block file into the environment.
See Environment block.

The --file option overrides the default location of the environment
block.

The --skip-sig option skips signature checking even when the
value of environment variable check_signatures is set to
enforce (see check_signatures).

If one or more variable names are provided as arguments, they are
interpreted as a whitelist of variables to load from the environment
block file. Variables set in the file but not present in the
whitelist are ignored.

The --skip-sig option should be used with care, and should
always be used in concert with a whitelist of acceptable variables
whose values should be set. Failure to employ a carefully constructed
whitelist could result in reading a malicious value into critical
environment variables from the file, such as setting
check_signatures=no, modifying prefix to boot from an
unexpected location or not at all, etc.

When used with care, --skip-sig and the whitelist enable an
administrator to configure a system to boot only signed
configurations, but to allow the user to select from among multiple
configurations, and to enable “one-shot” boot attempts and
“savedefault” behavior. See Using digital signatures, for more
information.

16.3.49 multiboot

Load a multiboot kernel image from file. The rest of the
line is passed verbatim as the kernel command-line. Any module must
be reloaded after using this command (see module).

Some kernels have known problems. You need to specify –quirk-* for those.
–quirk-bad-kludge is a problem seen in several products that they include
loading kludge information with invalid data in ELF file. GRUB prior to 0.97
and some custom builds prefered ELF information while 0.97 and GRUB 2
use kludge. Use this option to ignore kludge.
Known affected systems: old Solaris, SkyOS.

–quirk-modules-after-kernel is needed for kernels which load at relatively
high address e.g. 16MiB mark and can’t cope with modules stuffed between
1MiB mark and beginning of the kernel.
Known afftected systems: VMWare.

16.3.51 normal

Command: normal[file]

Enter normal mode and display the GRUB menu.

In normal mode, commands, filesystem modules, and cryptography modules are
automatically loaded, and the full GRUB script parser is available. Other
modules may be explicitly loaded using insmod (see insmod).

If a file is given, then commands will be read from that file.
Otherwise, they will be read from $prefix/grub.cfg if it exists.

normal may be called from within normal mode, creating a nested
environment. It is more usual to use configfile
(see configfile) for this.

16.3.53 parttool

Command: parttoolpartition commands

Make various modifications to partition table entries.

Each command is either a boolean option, in which case it must be
followed with ‘+’ or ‘-’ (with no intervening space) to enable or
disable that option, or else it takes a value in the form
‘command=value’.

Currently, parttool is only useful on DOS partition tables (also
known as Master Boot Record, or MBR). On these partition tables, the
following commands are available:

‘boot’ (boolean)

When enabled, this makes the selected partition be the active (bootable)
partition on its disk, clearing the active flag on all other partitions.
This command is limited to primary partitions.

‘type’ (value)

Change the type of an existing partition. The value must be a number in the
range 0-0xFF (prefix with ‘0x’ to enter it in hexadecimal).

‘hidden’ (boolean)

When enabled, this hides the selected partition by setting the hidden
bit in its partition type code; when disabled, unhides the selected
partition by clearing this bit. This is useful only when booting DOS or
Wwindows and multiple primary FAT partitions exist in one disk. See also
DOS/Windows.

16.3.56 play

Command: playfile | tempo [pitch1 duration1] [pitch2 duration2] …

Plays a tune

If the argument is a file name (see File name syntax), play the tune
recorded in it. The file format is first the tempo as an unsigned 32bit
little-endian number, then pairs of unsigned 16bit little-endian numbers for
pitch and duration pairs.

If the arguments are a series of numbers, play the inline tune.

The tempo is the base for all note durations. 60 gives a 1-second base, 120
gives a half-second base, etc. Pitches are Hz. Set pitch to 0 to produce
a rest.

16.3.62 rmmod

16.3.63 save_env

Save the named variables from the environment to the environment block file.
See Environment block.

The --file option overrides the default location of the environment
block.

This command will operate successfully even when environment variable
check_signatures is set to enforce
(see check_signatures), since it writes to disk and does not alter
the behavior of GRUB based on any contents of disk that have been
read. It is possible to modify a digitally signed environment block
file from within GRUB using this command, such that its signature will
no longer be valid on subsequent boots. Care should be taken in such
advanced configurations to avoid rendering the system
unbootable. See Using digital signatures, for more information.

Insert keystrokes into the keyboard buffer when booting. Sometimes an
operating system or chainloaded boot loader requires particular keys to be
pressed: for example, one might need to press a particular key to enter
"safe mode", or when chainloading another boot loader one might send
keystrokes to it to navigate its menu.

You may provide up to 16 keystrokes (the length of the BIOS keyboard
buffer). Keystroke names may be upper-case or lower-case letters, digits,
or taken from the following table:

Name

Key

escape

Escape

exclam

!

at

@

numbersign

#

dollar

$

percent

%

caret

^

ampersand

&

asterisk

*

parenleft

(

parenright

)

minus

-

underscore

_

equal

=

plus

+

backspace

Backspace

tab

Tab

bracketleft

[

braceleft

{

bracketright

]

braceright

}

enter

Enter

control

press and release Control

semicolon

;

colon

:

quote

’

doublequote

"

backquote

‘

tilde

~

shift

press and release left Shift

backslash

\

bar

|

comma

,

less

<

period

.

greater

>

slash

/

question

?

rshift

press and release right Shift

alt

press and release Alt

space

space bar

capslock

Caps Lock

F1

F1

F2

F2

F3

F3

F4

F4

F5

F5

F6

F6

F7

F7

F8

F8

F9

F9

F10

F10

F11

F11

F12

F12

num1

1 (numeric keypad)

num2

2 (numeric keypad)

num3

3 (numeric keypad)

num4

4 (numeric keypad)

num5

5 (numeric keypad)

num6

6 (numeric keypad)

num7

7 (numeric keypad)

num8

8 (numeric keypad)

num9

9 (numeric keypad)

num0

0 (numeric keypad)

numperiod

. (numeric keypad)

numend

End (numeric keypad)

numdown

Down (numeric keypad)

numpgdown

Page Down (numeric keypad)

numleft

Left (numeric keypad)

numcenter

5 with Num Lock inactive (numeric keypad)

numright

Right (numeric keypad)

numhome

Home (numeric keypad)

numup

Up (numeric keypad)

numpgup

Page Up (numeric keypad)

numinsert

Insert (numeric keypad)

numdelete

Delete (numeric keypad)

numasterisk

* (numeric keypad)

numminus

- (numeric keypad)

numplus

+ (numeric keypad)

numslash

/ (numeric keypad)

numenter

Enter (numeric keypad)

delete

Delete

insert

Insert

home

Home

end

End

pgdown

Page Down

pgup

Page Up

down

Down

up

Up

left

Left

right

Right

As well as keystrokes, the sendkey command takes various options
that affect the BIOS keyboard status flags. These options take an ‘on’
or ‘off’ parameter, specifying that the corresponding status flag be
set or unset; omitting the option for a given status flag will leave that
flag at its initial state at boot. The --num, --caps,
--scroll, and --insert options emulate setting the
corresponding mode, while the --numkey, --capskey,
--scrollkey, and --insertkey options emulate pressing and
holding the corresponding key. The other status flag options are
self-explanatory.

If the --no-led option is given, the status flag options will have
no effect on keyboard LEDs.

If the sendkey command is given multiple times, then only the last
invocation has any effect.

Since sendkey manipulates the BIOS keyboard buffer, it may cause
hangs, reboots, or other misbehaviour on some systems. If the operating
system or boot loader that runs after GRUB uses its own keyboard driver
rather than the BIOS keyboard functions, then sendkey will have no
effect.

16.3.70 sleep

Command: sleep[--verbose] [--interruptible] count

Sleep for count seconds. If option --interruptible is given,
allow ESC to interrupt sleep. With --verbose show countdown
of remaining seconds. Exit code is set to 0 if timeout expired and to 1
if timeout was interrupted by ESC.

16.3.71 source

Command: sourcefile

Read file as a configuration file, as if its contents had been
incorporated directly into the sourcing file. Unlike configfile
(see configfile), this executes the contents of file without
changing context: any environment variable changes made by the commands in
file will be preserved after source returns, and the menu
will not be shown immediately.

16.3.74 trust

Command: trust[--skip-sig] pubkey_file

Read public key from pubkey_file and add it to GRUB’s internal
list of trusted public keys. These keys are used to validate digital
signatures when environment variable check_signatures is set to
enforce. Note that if check_signatures is set to
enforce when trust executes, then pubkey_file
must itself be properly signed. The --skip-sig option can be
used to disable signature-checking when reading pubkey_file
itself. It is expected that --skip-sig is useful for testing
and manual booting. See Using digital signatures, for more
information.

16.3.77 verify_detached

Verifies a GPG-style detached signature, where the signed file is
file, and the signature itself is in file signature_file.
Optionally, a specific public key to use can be specified using
pubkey_file. When environment variable check_signatures
is set to enforce, then pubkey_file must itself be
properly signed by an already-trusted key. An unsigned
pubkey_file can be loaded by specifying --skip-sig.
If pubkey_file is omitted, then public keys from GRUB’s trusted keys
(see list_trusted, see trust, and see distrust) are
tried.

Exit code $? is set to 0 if the signature validates
successfully. If validation fails, it is set to a non-zero value.
See Using digital signatures, for more information.

16.4.1 net_add_addr

Command: net_add_addrinterfacecardaddress

Configure additional network interface with address on a
network card. address can be either IP in dotted decimal notation,
or symbolic name which is resolved using DNS lookup. If successful, this command
also adds local link routing entry to the default subnet of address
with name interface‘:local’ via interface.

16.4.2 net_add_dns

16.4.3 net_add_route

Command: net_add_routeshortnameip[/prefix] [interface | ‘gw’ gateway]

Add route to network with address ip as modified by prefix via
either local interface or gateway. prefix is optional and
defaults to 32 for IPv4 address and 128 for IPv6 address. Route is identified
by shortname which can be used to remove it (see net_del_route).

16.4.4 net_bootp

Command: net_bootp[card]

Perform configuration of card using DHCP protocol. If no card name
is specified, try to configure all existing cards. If configuration was
successful, interface with name card‘:dhcp’ and configured
address is added to card. If server provided gateway information in
DHCP ACK packet, it is added as route entry with the name card‘:dhcp:gw’. Additionally the following DHCP options are recognized and processed:

‘1 (Subnet Mask)’

Used to calculate network local routing entry for interface card‘:dhcp’.

‘3 (Router)’

Adds default route entry with the name card‘:dhcp:default’ via gateway
from DHCP option. Note that only option with single route is accepted.

‘6 (Domain Name Server)’

Adds all servers from option value to the list of servers used during name resolution.

16.4.7 net_del_route

16.4.8 net_get_dhcp_option

Command: net_get_dhcp_optionvarinterfacenumbertype

Request DHCP option number of type via interface. type
can be one of ‘string’, ‘number’ or ‘hex’. If option is found,
assign its value to variable var. Values of types ‘number’ and ‘hex’
are converted to string representation.

17 Internationalisation

17.1 Charset

GRUB uses UTF-8 internally other than in rendering where some GRUB-specific
appropriate representation is used. All text files (including config) are
assumed to be encoded in UTF-8.

17.2 Filesystems

NTFS, JFS, UDF, HFS+, exFAT, long filenames in FAT, Joliet part of
ISO9660 are treated as UTF-16 as per specification. AFS and BFS are read
as UTF-8, again according to specification. BtrFS, cpio, tar, squash4, minix,
minix2, minix3, ROMFS, ReiserFS, XFS, ext2, ext3, ext4, FAT (short names),
RockRidge part of ISO9660, nilfs2, UFS1, UFS2 and ZFS are assumed
to be UTF-8. This might be false on systems configured with legacy charset
but as long as the charset used is superset of ASCII you should be able to
access ASCII-named files. And it’s recommended to configure your system to use
UTF-8 to access the filesystem, convmv may help with migration. ISO9660 (plain)
filenames are specified as being ASCII or being described with unspecified
escape sequences. GRUB assumes that the ISO9660 names are UTF-8 (since
any ASCII is valid UTF-8). There are some old CD-ROMs which use CP437
in non-compliant way. You’re still able to access files with names containing
only ASCII characters on such filesystems though. You’re also able to access
any file if the filesystem contains valid Joliet (UTF-16) or RockRidge (UTF-8).
AFFS, SFS and HFS never use unicode and GRUB assumes them to be in Latin1,
Latin1 and MacRoman respectively. GRUB handles filesystem case-insensitivity
however no attempt is performed at case conversion of international characters
so e.g. a file named lowercase greek alpha is treated as different from
the one named as uppercase alpha. The filesystems in questions are
NTFS (except POSIX namespace), HFS+ (configurable at mkfs time, default
insensitive), SFS (configurable at mkfs time, default insensitive),
JFS (configurable at mkfs time, default sensitive), HFS, AFFS, FAT, exFAT
and ZFS (configurable on per-subvolume basis by property “casesensitivity”,
default sensitive). On ZFS subvolumes marked as case insensitive files
containing lowercase international characters are inaccessible.
Also like all supported filesystems except HFS+ and ZFS (configurable on
per-subvolume basis by property “normalization”, default none) GRUB makes
no attempt at check of canonical equivalence so a file name u-diaresis is
treated as distinct from u+combining diaresis. This however means that in
order to access file on HFS+ its name must be specified in normalisation form D.
On normalized ZFS subvolumes filenames out of normalisation are inaccessible.

17.3 Output terminal

Firmware output console “console” on ARC and IEEE1275 are limited to ASCII.

BIOS firmware console and VGA text are limited to ASCII and some pseudographics.

None of above mentioned is appropriate for displaying international and any
unsupported character is replaced with question mark except pseudographics
which we attempt to approximate with ASCII.

EFI console on the other hand nominally supports UTF-16 but actual language
coverage depends on firmware and may be very limited.

The encoding used on serial can be chosen with terminfo as
either ASCII, UTF-8 or “visual UTF-8”. Last one is against the specification
but results in correct rendering of right-to-left on some readers which don’t
have own bidi implementation.

On emu GRUB checks if charset is UTF-8 and uses it if so and uses ASCII
otherwise.

When using gfxterm or gfxmenu GRUB itself is responsible for rendering the
text. In this case GRUB is limited by loaded fonts. If fonts contain all
required characters then bidirectional text, cursive variants and combining
marks other than enclosing, half (e.g. left half tilde or combining overline)
and double ones. Ligatures aren’t supported though. This should cover European,
Middle Eastern (if you don’t mind lack of lam-alif ligature in Arabic) and
East Asian scripts. Notable unsupported scripts are Brahmic family and
derived as well as Mongolian, Tifinagh, Korean Jamo (precomposed characters
have no problem) and tonal writing (2e5-2e9). GRUB also ignores deprecated
(as specified in Unicode) characters (e.g. tags). GRUB also doesn’t handle so
called “annotation characters” If you can complete either of
two lists or, better, propose a patch to improve rendering, please contact
developer team.

17.4 Input terminal

Firmware console on BIOS, IEEE1275 and ARC doesn’t allow you to enter non-ASCII
characters. EFI specification allows for such but author is unaware of any
actual implementations. Serial input is currently limited for latin1 (unlikely
to change). Own keyboard implementations (at_keyboard and usb_keyboard)
supports any key but work on one-char-per-keystroke.
So no dead keys or advanced input method. Also there is no keymap change hotkey.
In practice it makes difficult to enter any text using non-Latin alphabet.
Moreover all current input consumers are limited to ASCII.

17.5 Gettext

GRUB supports being translated. For this you need to have language *.mo files in $prefix/locale, load gettext module and set “lang” variable.

17.6 Regexp

Regexps work on unicode characters, however no attempt at checking cannonical
equivalence has been made. Moreover the classes like [:alpha:] match only
ASCII subset.

17.7 Other

Currently GRUB always uses YEAR-MONTH-DAY HOUR:MINUTE:SECOND [WEEKDAY] 24-hour
datetime format but weekdays are translated.
GRUB always uses the decimal number format with [0-9] as digits and . as
descimal separator and no group separator.
IEEE1275 aliases are matched case-insensitively except non-ASCII which is
matched as binary. Similar behaviour is for matching OSBundleRequired.
Since IEEE1275 aliases and OSBundleRequired don’t contain any non-ASCII it
should never be a problem in practice.
Case-sensitive identifiers are matched as raw strings, no canonical
equivalence check is performed. Case-insenstive identifiers are matched
as RAW but additionally [a-z] is equivalent to [A-Z]. GRUB-defined
identifiers use only ASCII and so should user-defined ones.
Identifiers containing non-ASCII may work but aren’t supported.
Only the ASCII space characters (space U+0020, tab U+000b, CR U+000d and
LF U+000a) are recognised. Other unicode space characters aren’t a valid
field separator.
test (see test) tests <, >, <=, >=, -pgt and -plt compare the strings in the
lexicographical order of unicode codepoints, replicating the behaviour of
test from coreutils.
environment variables and commands are listed in the same order.

18 Security

18.1 Authentication and authorisation in GRUB

By default, the boot loader interface is accessible to anyone with physical
access to the console: anyone can select and edit any menu entry, and anyone
can get direct access to a GRUB shell prompt. For most systems, this is
reasonable since anyone with direct physical access has a variety of other
ways to gain full access, and requiring authentication at the boot loader
level would only serve to make it difficult to recover broken systems.

However, in some environments, such as kiosks, it may be appropriate to lock
down the boot loader to require authentication before performing certain
operations.

The ‘password’ (see password) and ‘password_pbkdf2’
(see password_pbkdf2) commands can be used to define users, each of
which has an associated password. ‘password’ sets the password in
plain text, requiring grub.cfg to be secure; ‘password_pbkdf2’
sets the password hashed using the Password-Based Key Derivation Function
(RFC 2898), requiring the use of grub-mkpasswd-pbkdf2
(see Invoking grub-mkpasswd-pbkdf2) to generate password hashes.

In order to enable authentication support, the ‘superusers’ environment
variable must be set to a list of usernames, separated by any of spaces,
commas, semicolons, pipes, or ampersands. Superusers are permitted to use
the GRUB command line, edit menu entries, and execute any menu entry. If
‘superusers’ is set, then use of the command line is automatically
restricted to superusers.

Other users may be given access to specific menu entries by giving a list of
usernames (as above) using the --users option to the
‘menuentry’ command (see menuentry). If the --unrestricted
option is used for a menu entry, then that entry is unrestricted.
If the --users option is not used for a menu entry, then that
only superusers are able to use it.

The grub-mkconfig program does not yet have built-in support for
generating configuration files with authentication. You can use
/etc/grub.d/40_custom to add simple superuser authentication, by
adding set superusers= and password or password_pbkdf2
commands.

18.2 Using digital signatures in GRUB

GRUB’s core.img can optionally provide enforcement that all files
subsequently read from disk are covered by a valid digital signature.
This document does not cover how to ensure that your
platform’s firmware (e.g., Coreboot) validates core.img.

If environment variable check_signatures
(see check_signatures) is set to enforce, then every
attempt by the GRUB core.img to load another file foo
implicitly invokes verify_detached foo foo.sig
(see verify_detached). foo.sig must contain a valid
digital signature over the contents of foo, which can be
verified with a public key currently trusted by GRUB
(see list_trusted, see trust, and see distrust). If
validation fails, then file foo cannot be opened. This failure
may halt or otherwise impact the boot process.

GRUB uses GPG-style detached signatures (meaning that a file
foo.sig will be produced when file foo is signed), and
currently supports the DSA and RSA signing algorithms. A signing key
can be generated as follows:

gpg --gen-key

An individual file can be signed as follows:

gpg --detach-sign /path/to/file

For successful validation of all of GRUB’s subcomponents and the
loaded OS kernel, they must all be signed. One way to accomplish this
is the following (after having already produced the desired
grub.cfg file, e.g., by running grub-mkconfig
(see Invoking grub-mkconfig):

Note that internally signature enforcement is controlled by setting
the environment variable check_signatures equal to
enforce. Passing one or more --pubkey options to
grub-mkimage implicitly defines check_signatures
equal to enforce in core.img prior to processing any
configuration files.

Note that signature checking does not prevent an attacker
with (serial, physical, ...) console access from dropping manually to
the GRUB console and executing:

set check_signatures=no

To prevent this, password-protection (see Authentication and authorisation) is essential. Note that even with GRUB password
protection, GRUB itself cannot prevent someone with physical access to
the machine from altering that machine’s firmware (e.g., Coreboot
or BIOS) configuration to cause the machine to boot from a different
(attacker-controlled) device. GRUB is at best only one link in a
secure boot chain.

19 Platform limitations

GRUB2 is designed to be portable and is actually ported across platforms. We
try to keep all platforms at the level. Unfortunately some platforms are better
supported than others. This is detailed in current and 2 following sections.

ARC platform is unable to change datetime (firmware doesn’t seem to provide a
function for it).
EMU has similar limitation.

On EMU platform no serial port is available.

Console charset refers only to firmware-assisted console. gfxterm is always
Unicode (see Internationalisation section for its limitations). Serial is
configurable to UTF-8 or ASCII (see Internationalisation). In case of qemu
and coreboot ports the refered console is vga_text. Loongson always uses
gfxterm.

Most limited one is ASCII. CP437 provides additionally pseudographics.
GRUB2 doesn’t use any language characters from CP437 as often CP437 is replaced
by national encoding compatible only in pseudographics.
Unicode is the most versatile charset which supports many languages. However
the actual console may be much more limited depending on firmware

On BIOS network is supported only if the image is loaded through network.
On sparc64 GRUB is unable to determine which server it was booted from.

Direct ATA/AHCI support allows to circumvent various firmware limitations but
isn’t needed for normal operation except on baremetal ports.

AT keyboard support allows keyboard layout remapping and support for keys not
available through firmware. It isn’t needed for normal operation except
baremetal ports.

Speaker allows morse and spkmodem communication.

USB support provides benefits similar to ATA (for USB disks) or AT (for USB
keyboards). In addition it allows USBserial.

Chainloading refers to the ability to load another bootloader through the same protocol

Hints allow faster disk discovery by already knowing in advance which is the disk in
question. On some platforms hints are correct unless you move the disk between boots.
On other platforms it’s just an educated guess.
Note that hint failure results in just reduced performance, not a failure

BadRAM is the ability to mark some of the RAM as “bad”. Note: due to protocol
limitations mips-loongson (with Linux protocol)
and mips-qemu_mips can use only memory up to first hole.

21 Supported boot targets

X86 support is summarised in the following table. “Yes” means that the kernel works on the given platform, “crashes” means an early kernel crash which we hope will be fixed by concerned kernel developers. “no” means GRUB doesn’t load the given kernel on a given platform. “headless” means that the kernel works but lacks console drivers (you can still use serial or network console). In case of “no” and “crashes” the reason is given in footnote.

BIOS

Coreboot

BIOS chainloading

yes

no (1)

NTLDR

yes

no (1)

Plan9

yes

no (1)

Freedos

yes

no (1)

FreeBSD bootloader

yes

crashes (1)

32-bit kFreeBSD

yes

crashes (5)

64-bit kFreeBSD

yes

crashes (5)

32-bit kNetBSD

yes

crashes (1)

64-bit kNetBSD

yes

crashes

32-bit kOpenBSD

yes

yes

64-bit kOpenBSD

yes

yes

Multiboot

yes

yes

Multiboot2

yes

yes

32-bit Linux (legacy protocol)

yes

no (1)

64-bit Linux (legacy protocol)

yes

no (1)

32-bit Linux (modern protocol)

yes

yes

64-bit Linux (modern protocol)

yes

yes

32-bit XNU

yes

?

64-bit XNU

yes

?

32-bit EFI chainloader

no (2)

no (2)

64-bit EFI chainloader

no (2)

no (2)

Appleloader

no (2)

no (2)

Multiboot

Qemu

BIOS chainloading

no (1)

no (1)

NTLDR

no (1)

no (1)

Plan9

no (1)

no (1)

FreeDOS

no (1)

no (1)

FreeBSD bootloader

crashes (1)

crashes (1)

32-bit kFreeBSD

crashes (5)

crashes (5)

64-bit kFreeBSD

crashes (5)

crashes (5)

32-bit kNetBSD

crashes (1)

crashes (1)

64-bit kNetBSD

yes

yes

32-bit kOpenBSD

yes

yes

64-bit kOpenBSD

yes

yes

Multiboot

yes

yes

Multiboot2

yes

yes

32-bit Linux (legacy protocol)

no (1)

no (1)

64-bit Linux (legacy protocol)

no (1)

no (1)

32-bit Linux (modern protocol)

yes

yes

64-bit Linux (modern protocol)

yes

yes

32-bit XNU

?

?

64-bit XNU

?

?

32-bit EFI chainloader

no (2)

no (2)

64-bit EFI chainloader

no (2)

no (2)

Appleloader

no (2)

no (2)

ia32 EFI

amd64 EFI

BIOS chainloading

no (1)

no (1)

NTLDR

no (1)

no (1)

Plan9

no (1)

no (1)

FreeDOS

no (1)

no (1)

FreeBSD bootloader

crashes (1)

crashes (1)

32-bit kFreeBSD

headless

headless

64-bit kFreeBSD

headless

headless

32-bit kNetBSD

crashes (1)

crashes (1)

64-bit kNetBSD

yes

yes

32-bit kOpenBSD

headless

headless

64-bit kOpenBSD

headless

headless

Multiboot

yes

yes

Multiboot2

yes

yes

32-bit Linux (legacy protocol)

no (1)

no (1)

64-bit Linux (legacy protocol)

no (1)

no (1)

32-bit Linux (modern protocol)

yes

yes

64-bit Linux (modern protocol)

yes

yes

32-bit XNU

yes

yes

64-bit XNU

yes (4)

yes

32-bit EFI chainloader

yes

no (3)

64-bit EFI chainloader

no (3)

yes

Appleloader

yes

yes

ia32 IEEE1275

BIOS chainloading

no (1)

NTLDR

no (1)

Plan9

no (1)

FreeDOS

no (1)

FreeBSD bootloader

crashes (1)

32-bit kFreeBSD

crashes (5)

64-bit kFreeBSD

crashes (5)

32-bit kNetBSD

crashes (1)

64-bit kNetBSD

?

32-bit kOpenBSD

?

64-bit kOpenBSD

?

Multiboot

?

Multiboot2

?

32-bit Linux (legacy protocol)

no (1)

64-bit Linux (legacy protocol)

no (1)

32-bit Linux (modern protocol)

?

64-bit Linux (modern protocol)

?

32-bit XNU

?

64-bit XNU

?

32-bit EFI chainloader

no (2)

64-bit EFI chainloader

no (2)

Appleloader

no (2)

Requires BIOS

EFI only

32-bit and 64-bit EFI have different structures and work in different CPU modes so it’s not possible to chainload 32-bit bootloader on 64-bit platform and vice-versa

21.1 Boot tests

As you have seen in previous chapter the support matrix is pretty big and some of the configurations are only rarely used. To ensure the quality bootchecks are available for all x86 targets except EFI chainloader, Appleloader and XNU. All x86 platforms have bootcheck facility except ieee1275. Multiboot, multiboot2, BIOS chainloader, ntldr and freebsd-bootloader boot targets are tested only with a fake kernel images. Only Linux is tested among the payloads using Linux protocols.

22 Error messages produced by GRUB

22.1 GRUB only offers a rescue shell

GRUB’s normal start-up procedure involves setting the ‘prefix’
environment variable to a value set in the core image by
grub-install, setting the ‘root’ variable to match, loading
the ‘normal’ module from the prefix, and running the ‘normal’
command (see normal). This command is responsible for reading
/boot/grub/grub.cfg, running the menu, and doing all the useful
things GRUB is supposed to do.

If, instead, you only get a rescue shell, this usually means that GRUB
failed to load the ‘normal’ module for some reason. It may be possible
to work around this temporarily: for instance, if the reason for the failure
is that ‘prefix’ is wrong (perhaps it refers to the wrong device, or
perhaps the path to /boot/grub was not correctly made relative to the
device), then you can correct this and enter normal mode manually:

# Inspect the current prefix (and other preset variables):
set
# Find out which devices are available:
ls
# Set to the correct value, which might be something like this:
set prefix=(hd0,1)/grub
set root=(hd0,1)
insmod normal
normal

However, any problem that leaves you in the rescue shell probably means that
GRUB was not correctly installed. It may be more useful to try to reinstall
it properly using grub-install device (see Invoking grub-install). When doing this, there are a few things to remember:

Drive ordering in your operating system may not be the same as the boot
drive ordering used by your firmware. Do not assume that your first hard
drive (e.g. ‘/dev/sda’) is the one that your firmware will boot from.
device.map (see Device map) can be used to override this, but it
is usually better to use UUIDs or file system labels and avoid depending on
drive ordering entirely.

At least on BIOS systems, if you tell grub-install to install GRUB
to a partition but GRUB has already been installed in the master boot
record, then the GRUB installation in the partition will be ignored.

If possible, it is generally best to avoid installing GRUB to a partition
(unless it is a special partition for the use of GRUB alone, such as the
BIOS Boot Partition used on GPT). Doing this means that GRUB may stop being
able to read its core image due to a file system moving blocks around, such
as while defragmenting, running checks, or even during normal operation.
Installing to the whole disk device is normally more robust.

Check that GRUB actually knows how to read from the device and file system
containing /boot/grub. It will not be able to read from encrypted
devices with unsupported encryption scheme, nor from file systems for which
support has not yet been added to GRUB.

23 Invoking grub-install

The program grub-install generates a GRUB core image using
grub-mkimage and installs it on your system. You must specify the
device name on which you want to install GRUB, like this:

grub-install install_device

The device name install_device is an OS device name or a GRUB
device name.

grub-install accepts the following options:

--help

Print a summary of the command-line options and exit.

--version

Print the version number of GRUB and exit.

--boot-directory=dir

Install GRUB images under the directory dir/grub/
This option is useful when you want to install GRUB into a
separate partition or a removable disk.
If this option is not specified then it defaults to /boot, so

grub-install /dev/sda

is equivalent to

grub-install --boot-directory=/boot/ /dev/sda

Here is an example in which you have a separate boot partition which is
mounted on
/mnt/boot:

grub-install --boot-directory=/mnt/boot /dev/sdb

--recheck

Recheck the device map, even if /boot/grub/device.map already
exists. You should use this option whenever you add/remove a disk
into/from your computer.

--no-rs-codes

By default on x86 BIOS systems, grub-install will use some
extra space in the bootloader embedding area for Reed-Solomon
error-correcting codes. This enables GRUB to still boot successfully
if some blocks are corrupted. The exact amount of protection offered
is dependent on available space in the embedding area. R sectors of
redundancy can tolerate up to R/2 corrupted sectors. This
redundancy may be cumbersome if attempting to cryptographically
validate the contents of the bootloader embedding area, or in more
modern systems with GPT-style partition tables (see BIOS installation) where GRUB does not reside in any unpartitioned space
outside of the MBR. Disable the Reed-Solomon codes with this option.

27 Invoking grub-mkrescue

All arguments not explicitly listed as grub-mkrescue options are
passed on directly to xorriso in mkisofs emulation mode.
Options passed to xorriso will normally be interpreted as
mkisofs options; if the option ‘--’ is used, then anything
after that will be interpreted as native xorriso options.

Non-option arguments specify additional source directories. This is
commonly used to add extra files to the image:

28 Invoking grub-mount

The program grub-mount performs a read-only mount of any file
system or file system image that GRUB understands, using GRUB’s file system
drivers via FUSE. (It is only available if FUSE development files were
present when GRUB was built.) This has a number of uses:

It provides a convenient way to check how GRUB will view a file system at
boot time. You can use normal command-line tools to compare that view with
that of your operating system, making it easy to find bugs.

It offers true read-only mounts. Linux does not have these for journalling
file systems, because it will always attempt to replay the journal at mount
time; while you can temporarily mark the block device read-only to avoid
this, that causes the mount to fail. Since GRUB intentionally contains no
code for writing to file systems, it can easily provide a guaranteed
read-only mount mechanism.

It allows you to examine any file system that GRUB understands without
needing to load additional modules into your running kernel, which may be
useful in constrained environments such as installers.

Since it can examine file system images (contained in regular files) just as
easily as file systems on block devices, you can use it to inspect any file
system image that GRUB understands with only enough privileges to use FUSE,
even if nobody has yet written a FUSE module specifically for that file
system type.

Using grub-mount is normally as simple as:

grub-mount /dev/sda1 /mnt

grub-mount must be given one or more images and a mount point as
non-option arguments (if it is given more than one image, it will treat them
as a RAID set), and also accepts the following options:

--help

Print a summary of the command-line options and exit.

--version

Print the version number of GRUB and exit.

-C

--crypto

Mount encrypted devices, prompting for a passphrase if necessary.

-d string

--debug=string

Show debugging output for conditions matching string.

-K prompt|file

--zfs-key=prompt|file

Load a ZFS encryption key. If you use ‘prompt’ as the argument,
grub-mount will read a passphrase from the terminal; otherwise, it
will read key material from the specified file.

-r device

--root=device

Set the GRUB root device to device. You do not normally need to set
this; grub-mount will automatically set the root device to the
root of the supplied file system.

If device is just a number, then it will be treated as a partition
number within the supplied image. This means that, if you have an image of
an entire disk in disk.img, then you can use this command to mount
its second partition:

29 Invoking grub-probe

grub-probe must be given a path or device as a non-option
argument, and also accepts the following options:

--help

Print a summary of the command-line options and exit.

--version

Print the version number of GRUB and exit.

-d

--device

If this option is given, then the non-option argument is a system device
name (such as ‘/dev/sda1’), and grub-probe will print
information about that device. If it is not given, then the non-option
argument is a filesystem path (such as ‘/boot/grub’), and
grub-probe will print information about the device containing that
part of the filesystem.

-m file

--device-map=file

Use file as the device map (see Device map) rather than the
default, usually ‘/boot/grub/device.map’.

-t target

--target=target

Print information about the given path or device as defined by target.
The available targets and their meanings are:

‘fs’

GRUB filesystem module.

‘fs_uuid’

Filesystem Universally Unique Identifier (UUID).

‘fs_label’

Filesystem label.

‘drive’

GRUB device name.

‘device’

System device name.

‘partmap’

GRUB partition map module.

‘abstraction’

GRUB abstraction module (e.g. ‘lvm’).

‘cryptodisk_uuid’

Crypto device UUID.

‘msdos_parttype’

MBR partition type code (two hexadecimal digits).

‘hints_string’

A string of platform search hints suitable for passing to the
search command (see search).

‘bios_hints’

Search hints for the PC BIOS platform.

‘ieee1275_hints’

Search hints for the IEEE1275 platform.

‘baremetal_hints’

Search hints for platforms where disks are addressed directly rather than
via firmware.

‘efi_hints’

Search hints for the EFI platform.

‘arc_hints’

Search hints for the ARC platform.

‘compatibility_hint’

A guess at a reasonable GRUB drive name for this device, which may be
used as a fallback if the search command fails.

30 Invoking grub-script-check

The program grub-script-check takes a GRUB script file
(see Shell-like scripting) and checks it for syntax errors, similar to
commands such as sh -n. It may take a path as a non-option
argument; if none is supplied, it will read from standard input.

Appendix A How to obtain and build GRUB

Caution: GRUB requires binutils-2.9.1.0.23 or later because the
GNU assembler has been changed so that it can produce real 16bits
machine code between 2.9.1 and 2.9.1.0.x. See
http://sources.redhat.com/binutils/, to obtain information on
how to get the latest version.

GRUB is available from the GNU alpha archive site
ftp://ftp.gnu.org/gnu/grub or any of its mirrors. The file
will be named grub-version.tar.gz. The current version is
2.02~beta2, so the file you should grab is:

which will create a directory called grub-2.02~beta2 with
all the sources. You can look at the file INSTALL for detailed
instructions on how to build and install GRUB, but you should be able to
just do:

Appendix B Reporting bugs

These are the guideline for how to report bugs. Take a look at this
list below before you submit bugs:

Before getting unsettled, read this manual through and through. Also,
see the GNU GRUB FAQ.

Always mention the information on your GRUB. The version number and the
configuration are quite important. If you build it yourself, write the
options specified to the configure script and your operating system,
including the versions of gcc and binutils.

If you have trouble with the installation, inform us of how you
installed GRUB. Don’t omit error messages, if any. Just ‘GRUB hangs
up when it boots’ is not enough.

The information on your hardware is also essential. These are especially
important: the geometries and the partition tables of your hard disk
drives and your BIOS.

If GRUB cannot boot your operating system, write down
everything you see on the screen. Don’t paraphrase them, like
‘The foo OS crashes with GRUB, even though it can boot with the
bar boot loader just fine’. Mention the commands you executed, the
messages printed by them, and information on your operating system
including the version number.

Explain what you wanted to do. It is very useful to know your purpose
and your wish, and how GRUB didn’t satisfy you.

If you can investigate the problem yourself, please do. That will give
you and us much more information on the problem. Attaching a patch is
even better.

When you attach a patch, make the patch in unified diff format, and
write ChangeLog entries. But, even when you make a patch, don’t forget
to explain the problem, so that we can understand what your patch is
for.

Write down anything that you think might be related. Please understand
that we often need to reproduce the same problem you encountered in our
environment. So your information should be sufficient for us to do the
same thing—Don’t forget that we cannot see your computer directly. If
you are not sure whether to state a fact or leave it out, state it!
Reporting too many things is much better than omitting something
important.

If you follow the guideline above, submit a report to the
Bug Tracking System.
Alternatively, you can submit a report via electronic mail to
bug-grub@gnu.org, but we strongly recommend that you use the
Bug Tracking System, because e-mail can be passed over easily.

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VERBATIM COPYING

You may copy and distribute the Document in any medium, either
commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no other
conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document’s license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify
you as the publisher of these copies. The front cover must present
the full title with all words of the title equally prominent and
visible. You may add other material on the covers in addition.
Copying with changes limited to the covers, as long as they preserve
the title of the Document and satisfy these conditions, can be treated
as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
(which should, if there were any, be listed in the History section
of the Document). You may use the same title as a previous version
if the original publisher of that version gives permission.

List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified
Version, together with at least five of the principal authors of the
Document (all of its principal authors, if it has fewer than five),
unless they release you from this requirement.

State on the Title page the name of the publisher of the
Modified Version, as the publisher.

Preserve all the copyright notices of the Document.

Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.

Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the
terms of this License, in the form shown in the Addendum below.

Preserve in that license notice the full lists of Invariant Sections
and required Cover Texts given in the Document’s license notice.

Include an unaltered copy of this License.

Preserve the section Entitled “History”, Preserve its Title, and add
to it an item stating at least the title, year, new authors, and
publisher of the Modified Version as given on the Title Page. If
there is no section Entitled “History” in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.

Preserve the network location, if any, given in the Document for
public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions
it was based on. These may be placed in the “History” section.
You may omit a network location for a work that was published at
least four years before the Document itself, or if the original
publisher of the version it refers to gives permission.

For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.

Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers
or the equivalent are not considered part of the section titles.

Delete any section Entitled “Endorsements”. Such a section
may not be included in the Modified Version.

Do not retitle any existing section to be Entitled “Endorsements” or
to conflict in title with any Invariant Section.

Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version’s license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled “Endorsements”, provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,
you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled “History”
in the various original documents, forming one section Entitled
“History”; likewise combine any sections Entitled “Acknowledgements”,
and any sections Entitled “Dedications”. You must delete all
sections Entitled “Endorsements.”

COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in
the collection, provided that you follow the rules of this License for
verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute
it individually under this License, provided you insert a copy of this
License into the extracted document, and follow this License in all
other respects regarding verbatim copying of that document.

AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an “aggregate” if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation’s users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document’s Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.

TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled “Acknowledgements”,
“Dedications”, or “History”, the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

TERMINATION

You may not copy, modify, sublicense, or distribute the Document except
as expressly provided for under this License. Any other attempt to
copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However,
parties who have received copies, or rights, from you under this
License will not have their licenses terminated so long as such
parties remain in full compliance.

FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time. Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License “or any later version” applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.

D.1.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

Copyright (C) yearyour name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the “with...Texts.” line with this:

with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.